A COMPEND OF THE DISEASES AND REFRACTION OF THE EYE. PART I. REFRACTION OF THE EYE. Anatomy of the Eye.-It is, of course, unnecessary to say that all proper treatment of the refractive errors, or of the diseases, of the eye, presupposes and demands a knowledge of its anatomy. Want of space compels us to omit a preliminary sketch of these details, and this is the less reprehensible because such an abstract already exists in the Quiz-Compend on Anatomy issued by the same publishers as is this volume; the student is therefore referred to that for a statement of the main facts of the anatomy of the orbital bones, the ocular muscles, and the structures of the globe itself. Résumé of Optical Principles.-Light and color, properly speaking, do not exist outside of the mind; they are sensations, creations by the brain upon its receipt of optic nerve messages from the retina. It is also more than probable that the neural vibration itself is not directly connected with the purely physical stimulus, but is the result of a molecular activity aroused by that stimulus in an intermediate receptive substance. We thus see that there are several transmutations of force between the external visible object and the phenomenon of consciousness we call light, but to avoid circumlocution we all speak of the physical cause of light as light itself, though scientifically speaking we are well aware of the absurdity. The molecules or atoms of a body, so long as that body is of a tempera ture below 525° C., have only that amplitude and rapidity of vibration which induce in the ether pressing about them the ethereal wave-motion we call radiant heat; that is, the ethereal oscillations are of a lesser frequency than 392 millions of millions per second. So soon as the temperature of a body rises above 525° C., it becomes (in the dark) visible, or self-luminous; its wave frequency rises above that of the numbers given, and we have the sensation red when looking upon it. With every increase of heat the vibrations of its molecules increase their rates of movement till the resultant ethereal movements reach a rapidity of arrival at the retina expressed by the figures 757,000,000,000,000. Above these frequencies we know the ether vibrates in higher and higher numbers, but these quicker movements produce no effect on the eye. The mind interprets as color the different rates of frequency between the two before-mentioned limits, and the combined effect of all frequencies (in the proportions furnished by sunlight) produces the sensation of dazzling white. Common sunlight, therefore, contains a mixture of all wave-lengths and frequencies, including the dark or Invisible Heat Rays, the Light Rays, and the Ultra-violet or Actinic Rays, as the upper invisible portion is called. Wave-length, wavefrequency and wave-refrangibility (the bending they undergo in passing into a denser medium) are all proportional to one another. The greater the wave-length the less both its frequency and its refrangibility. A prism, or a diffraction grating, ranges the homogeneous rays, massed together in a compound ray of sunlight, in a band, according to their refrangibility; this band, or ribbon of colors, is called the spectrum. By accurately delimiting the colors produced we find that daylight is made up of the following proportions of elemental colors: Red, 54 parts; orange-red, 140; orange, 80; orange-yellow, 114; yellow, 54; greenish-yellow, 206; yellowish-green, 121; green, and blue-green, 134; cyan-blue, 32; blue, 40; ultramarine, and blue-violet, 20; violet, 5; total, 1000. Again, all objects below a temperature of 525° C. become visible only by reflected light, and the colors of these objects are such as they are, because the light of day is partially absorbed by them, and partially reflected. If a body absorbs the rays of that frequency producing a certain color, it is of course, not of that color, but appears of that tint which is produced by the mixture of the reflected waves. The Fraunhofer lines of the spectrum are dark striæ caused by the arrestation of rays of certain frequencies by molecules in the sun's atmosphere, having the same vibrational periods. The following table gives the wave frequencies and corresponding lengths of the chief Fraunhofer lines and colors of the visible spectrum: These and all other wave frequencies pass through space at a common speed, 186,680 miles per second. So long as light traverses a homogeneous medium it proceeds in straight lines, but upon striking a (transparent) medium of greater density—as for instance, in passing from air into water -it is deflected from its preceding course, as is shown in the accompanying figure 2, when the incident ray E O, instead of proceeding to F is bent out of its course and takes the direction O W. Were the refracting medium glass, the refraction of E O would be greater, represented by the course of the line O G; were the medium diamond, for example, O D would be the direction of the bended ray. The angle of incidence is E O C, the angles of refraction, W O C, GOC, DOC. The sine of the angle of incidence e e', bears a constant ratio to the sines of the angles of refraction w w', gg', dd', and this ratio is called the Index of Refraction. For water, this index is 1.336, i.e., the sine e e' is to the sine ww', as 1.336 is to I, or about 1 to 1, or 4 to 3. The Index of Refraction of flint glass is about 1.6, or 8 to 5, of diamond, 5 to 1. It will be noticed the direction of the refracted ray always inclines toward the perpendicular, C C', and this fact gives us the second of the |