means of the data in Table XIII.

In this table I have collected the

chief results of my experiments. By means of Gray's values I have reduced the observed differences to a temperature of 15° C., and then calculated the values in vacuo. The curve thus obtained is more uniform in its course than the curves for air.

Table XIII.-Summary of Experiments with fresh Acid Solutions. (Tables IV-XII.)

The results of my experiments may be briefly summarised :1. With two copper voltameters containing freshly made neutral solution of copper sulphate, one of which is under reduced pressure, the copper deposit in the partial vacuum is higher (for the same current, current density, and temperature) than the deposit under the atmospheric pressure; but the percentage difference is not constant.

2. If a little free sulphuric acid be added to the air solution, the percentage difference is more constant and higher than in 1.

3. The addition of acid to both voltameters causes the percentage difference to be constant within experimental errors. The experiments conducted under this condition show that

i. For current densities above 0:01 ampère per square centimetre of active cathode, there is no practical difference between the two deposits.

ii. For current densities below 0.01 ampère per square centimetre, the vacuum deposit is very appreciably higher than the air deposit.

iii. A curve drawn representing the deposits obtained in vacuo at different current densities is more regular than the air curves, and for densities below 0:01 ampère per square centimetre is approximately a straight line.

"Note on the Action of Copper Sulphate and Sulphuric Acid on Metallic Copper." By ARTHUR SCHUSTER, F.R.S. Received November 14,-Read December 7, 1893.

Mr. Gannon in the foregoing paper refers to some unpublished experiments made by me a few years ago. These experiments were conducted for the purpose of satisfying myself that, as seemed a priori probable, the diminution of weight observed when metallic copper is exposed to the action of sulphuric acid or sulphate of copper is due to the presence of free oxygen in the liquid. Copper gauze was taken in order to deal with as large a surface as possible, and rolled up so as to fit into a piece of glass tubing. After the copper had been carefully washed, dried, and ignited in hydrogen, it was immersed in dilute sulphuric acid, the air above the acid was removed as far as possible, and the tube containing the gauze was then sealed hermetically. At the end of a fortnight a few tubes prepared in this way were opened and weighed after being subjected to exactly the same treatment as previous to immersion, that is to say, the copper was washed, dried, and ignited in hydrogen. The diminution in weight observed under these circumstances was insignificant. I cannot, unfortunately, now find the record of the actual weighings, but the quantities involved were about the same as in the next set of experiments. On January 26, 1891, four spirals of copper gauze were placed in a solution containing 20 per cent. of cupric sulphate, 5 or 10 per cent. by weight of sulphuric acid being added to the aqueous solution. The conditions thus approximated to the solutions which are used in the electrolysis of copper. The tubes were exhausted and sealed up: two of them were opened on February 2, and the two remaining ones on February 9; the weighings were taken after drying and ignition in hydrogen. The results are shown in the accompanying table:

It will be seen that the diminution in weight is quite insignificant compared to what takes place in the presence of air, and may be due to some remnant of oxygen left. The late Mr. Hoskyns Abrahall, however, suggested that it might also be due to the formation of copper sulphide; and this suggestion was supported by the fact that traces of sulphuretted hydrogen were given up when the copper, after

Insect Sight and the Defining Power of Composite Eyes. 85 immersion, was heated in hydrogen. The action would be represented by the formula

The above experiments prove that nearly the whole effect which is observed when copper is immersed in a solution of sulphate of copper or sulphuric acid is due to the presence of oxygen in the solution.

February 1, 1894.

Sir JOHN EVANS, K.C.B., D.C.L., LL.D., Vice-President and
Treasurer, in the Chair.

A List of the Presents received was laid on the table, and thanks ordered for them.

I. "Insect Sight and the Defining Power of Composite Eyes." By A. MALLOCK. Communicated by LORD RAYLEIGH, Sec. R.S. Received November 28, 1893.

The optical arrangement of the simple eyes of Vertebrates is well understood, but as regards the action of the composite eyes of Insects and Crustacea less certainty has hitherto prevailed.

In the former class of eye a single lens, or its equivalent, forms an image on a concave retina, built up, as a sort of tesselated pavement, of the sensitive terminations of the fibres of the optic nerve, and, if the lens is perfect and the pupil large enough, the definition is limited by the distance apart of the nerve-terminations, for, in order that two objects may appear as two to the eye, they must subtend at least such an angle that their images as formed by the lens shall not fall on the same nerve-termination.

In the human eye the distance between the sensitive points on the retina is such that it subtends about a minute of arc at the optic centre of the lens, and in good eyes the optical part of the apparatus is sufficiently perfect to allow of this degree of definition being attained over a small part of the field of view.

For reasons, however, which will be given presently, such definition as this is not to be looked for in composite eyes.

The general plan on which all composite eyes are constructed is that of a convex retina having a separate small lens in front of each sensitive part, together with an arrangement of screens which allows

only that light coming from the immediate neighbourhood of the axis of the lens to reach the nerve.

The theory of "mosaic vision" put forward by Johannes Müller has been opposed by some physiologists, who appear to have considered that each lens of a composite eye formed a complete image which was taken cognizance of by the nerves as in the vertebrate eye, and that the whole of these images were in some way added together and arranged by the brain; I here bring forward some optical reasons which show that Müller's view is the true one.

On the supposition, therefore, of "one lens, one impression," the definition obtained by a composite eye will be measured by the total solid angle of view ÷ whole number of lenses in the eye.

The simplest form of composite eye would be a spherical shell, AB, fig. 1, perforated with radial holes, c, c, c, the diameter of these

holes being small compared with the thickness of the shell.

If sensitive paper were placed in contact with the inner surface of the shell, it would be impressed with a picture of surrounding objects, for the light which reaches the bottom of any hole is limited to that making an angle less than DEF with the axis of the hole, which angle is of course equal to the diameter of the hole ÷ half its length. It is interesting to see what proportions would have to be given to an eye of this kind if the definition is to be as good as that of the human eye.

The limit of definition in this case being 1 min., the holes would have to be 7000 diameters long (since 1 min. is nearly 1/3500) and in order that diffraction may not interfere materially with the result,* the

=

*It may be shown that the hole should not be much smaller than the first Huyghens zone of a system for which, if λ/r = r/R, R the length of the hole, A and r being the wave-length of light and the radius of the zone respectively. How

diameter of the holes should not be less then 2000 wave-lengths of light, sayin. Hence the thickness of the shell will be 7000

in., or 23 ft.

The radius of the sphere may be determined by the condition that, if the picture is to be continuous, the adjacent holes must just be in contact at the internal surface of the shell, that is to say the diameter of the hole, viz.: in., must subtend 1 min. at the internal radius of the shell, which makes this radius, therefore, 11 ft. 6 in.

Thus an eye of this construction and power of definition would consist of some part of a spherical shell of 34 ft. 6 ins. external radius, and 23 ft. thick, perforated with radial holes in. in diameter, and with their centres about apart on the external surface.

If still keeping 1 min. as the limit of definition, we substitute the arrangement actually found in composite eyes, and in place of the long tunnels in thick shell, we use short tunnels with a lens at the outer end of each tunnel, and a diaphragm at the inner end, pierced with a small central hole (fig. 2), the proportion of the eye will be

determined in the first place by the diameter of the lens which will just define 1 min., and secondly by making that diameter subtend 1 min. at the centre of the sphere.

Now the size of the image of a point formed by a lens (as seen from the optic centre of the lens) is inversely as the diameter of the lens, and it takes a lens 4 ins. in diameter to define 1 second, i.e., to separate points 1" apart; hence the lens which will just define 1 min. is or 0.066 in. in diameter.

The radius at which 0·066 in. subtends 1 min. is about 19 ft.

It is evident, therefore, that no composite eye of practicable dimensions, acting as supposed above, could be made to give definition even approaching that of the human eye.

much less than the diameter of the hole may be is, to some extent, a matter of judgment depending on the degree to which it is considered desirable to reduce the intensity of the diffracted light.