PECULIARITIES OF VISION.] 



UNDULATORY FORCES. LIGHT. 



49 



from the face, and observe the effect produced. It will 

 be seen that, as the finger retrogrades, the two images, 

 previously separated, will gradually merge into one, so 

 that, at a distance of six inches from the nose, only one 

 image will be observed. In fact, a gradual junction of 

 the two images is thus effected. 



Again, bring the eyeballs into such a position that all 

 distant objects appear double just, in fact, so that the 

 axes of both eyes shall be directed to a point close to the 

 line parallel with the nose. If the finger be now held at 

 a distance of two feet from the eyes, it will appear 

 double : if, however, it be gradually brought so as nearly 

 to touch the nose, its two images will gradually coalesce, 

 and one image will be formed in the same place as that 

 in which two images were observed in our last experi- 

 ment. In this case we have brought, by the process of 

 "squinting" (as it is sometimes termed), the two axes or 

 lines of vision of the eye, so as to join each other at 

 a distance much less than that adopted in ordinary sight, 

 and have so produced an inversion of effect. In either 

 case, however, the cause was the same; namely, the 

 overlapping of the two images impressed on the retina, 

 but at different distances from a point situated midway 

 between the two eyes. 



We may here explain two instances of defective sight, 

 which we observe many persons to be troubled with. 

 They are those of long and short sight. In aged persons, 

 we find that the shape of the cornea has become altered, 



and less convex than is the case in a healthy state of the 

 organ. The remedy for this is very simple ; for what is 

 deficient in the eye is easily supplied by means of convex 

 lenses ; hence the general use of spectacles by old persons 

 and others, in whom the cornea is deficient in convexity. 

 Tbia state of the eye permits a person to see well enough 

 any distant object, because the rays proceeding thence 

 are comparatively parallel to those emanating from an 

 object nearer to the eye. 



In the case of "short-sighted" persons, the cornea is 

 far more convex than in ordinary cases. The use of con- 

 cave glasses is therefore essential to such, and they at 

 once supply the requisite remedy. 



We shall speak more fully on this subject when we 

 treat on optical instruments ; and shall defer the con- 

 sideration of some peculiarities of vision, in reference to 

 colour, to our section on the phenomena of colour. 



In many of the mammalia, the uses of the eye being, 

 to some extent, similar to those of the human subject, 

 its construction is mostly similar. Considerable suscepti- 

 bility to the action of light, in some animals, is generally 

 attended by a greater irritability of the iris ; and thus we 

 find those creatures which have their type in the common 

 cat, pones* better power of sight during dusk than at 

 other times. As we have already remarked, the pupil 

 of the eye contracts during strong daylight, and expands 

 as the light becomes less intense. The same adaptation 

 of organism to special circumstances is found in the owl, 

 Ac., amongst birds. 



The eye of the fish is peculiarly adapted to the condi- 

 tion of its existence, and may be taken as a kind of anti- 

 type of the organs of sight of insects. The fish naturally 

 exists in a liquid whose refractive power is scarcely 

 different to that of its organs of sight. In the insect we 

 find a most remarkable construction of the eye ; and of 

 this class we may refer to the eyes of the bee and the fly, 

 which are formed of a multitude of lenses. In general, 

 we may state, that the shape of the cornea is made to 

 depend on the circumstances in which the creature has 

 to maintain its existence. Thus, fishes have the cornea 

 flat, and the crystalline lens more spherical ; whilst the 

 cornea of the eye of the bird has greatet convexity, and 

 the crystalline lens is less convex than that of other 

 creatures. 



There are some interesting facts relating to the con- 

 itmction of the human eye as indicating design, which 

 we must not omit to place prominently before our 

 readers. Our remarks on aberration* will prepare us to 

 expect that special provision has been made to prevent 

 the consequences of that defect which causes so much 



* rn.it, f. 44. 

 YOl. I. 



loss and diffusion of light in spherical lenses. In the 

 human eye, the curvature of the cornea and crystalline 

 lens is such as entirely to prevent any chance of such a 

 defect existing; they, in fact, being elliptic, and hence 

 exactly of that form as would be required for a non- 

 aberrant lens. The voluntary motion of the iris regulates 

 the amount of light which can enter the eye. The change 

 of the convexity of the cornea provides for a distant or 

 close view of any object at pleasure. The mobility of 

 the eyeball enables us to instantly change the direction 

 of sight. The refractive powers of the humours of the 

 eye are exactly such as are required for the medium in 

 which we live. External to the eye, we have the pro- 

 tection of the eyelashes, which serve as a screen to 

 preserve the organ from the intrusion of foreign matter 

 on its surface. The prevalent colour of both the air 

 and the surface of the landscape, is just that which 

 agrees with the optical necessities of sight. Whilst the 

 ear cannot be closed to external sound, the eye, which is 

 an infinitely more delicate organ, can be shut off from 

 external impression, by the motion of the eyelids, at any 

 moment, and thus injury to its delicate organisation may 

 be almost entirely prevented. In no point of view, 

 therefore, is perfection and completeness more evident 

 than in the arrangement of the different parts of the eye 

 of man. 



It may also be remarked, that the eye is less subject 

 to disease, in general, than any other organ of the human 

 system. Despite its constant use, and apparent exposure 

 to external injury, we find, on an average of instances, 

 that it stands the wear and tear of life to an astonishing 

 extent. The changes of the healthy organ are very 

 gradual; and thus it is only towards the close of life, 

 when all the faculties and senses become dulled by age, 

 that the power of sight becomes seriously affected. Even 

 then, science gives to a "greun old age" a "second 

 sight" the very disease, indeed, suggesting its own 

 remedy. Whilst, during life, the eye is the exponent of 

 the sentiments of the mind alternately beaming with 

 pleasure or dimmed with pain it generally maintains 

 vitality to the closing scene, and, by its last glance, indi- 

 cates the farewell of its possessor to the changes and 

 shadows of earthly existence. 



THE PHENOMENA OF COLOUR. 



HAVTXO investigated the principal laws of the reflection 

 and refraction of light, we shall now enter on the con- 

 sideration of a most interesting branch of optical science ; 

 namely, the cause of colour. 



We have already stated that white light is composed of 

 certain rays which have been called primary colours ; and 

 we shall presume that the reader has become acquainted 

 with these, by examining the spectrum produced from a 

 ray of light refracted by means of a glass prism. We, 

 however, must remark, that the characteristics of a spec- 

 trum vary according to the substances employed as a 

 prism ; and by having a triangular vessel arranged so that 

 it can be filled with liquids of various refracting powers, 

 the student will soon discover, by its use, the results to 

 which we here refer. 



The chief effect produced by employing various sub- 

 stances in place of glass as a prism, is, that some have a 

 greater dispersive power over the rays of light passing 

 through them than others. Thus, a prism of flint-glass 

 increases the extent of the image of the coloured rays 

 much beyond that produced by one of crown-glass. And 

 the same may be stated of many other substances. Some 

 have the power of extending certain of the spectral rays 

 in preference to others, and of thus having a special action 

 on one or other end of the prismatic image. 



We have already stated that the different rays of the 

 spectrum may be divided into three kinds the luminous, 

 the calorific, and those having a chemical action on cer- 

 tain bodies. The chief of the luminous rays are found 

 in the yellow ; the greatest heating effect in the red ; and 

 the chemical power exists entirely in or beyond the blue 

 band. By placing a thermometer in the various bands 

 of colour, the student will be enabled at once to perceive 



H 



