14 GOKHAM, ON THE 



14. The pencils of light which are used for this purpose 

 not only illuminate the object, but intersect in their passage 

 through it, producing as many images as there are apertures. 



15. Hence, when a small object is examined by the light 

 admitted through small apertures, it will appear magnified, 

 inverted, multijjlied, and illuminated with variable degrees of 

 intensity. 



16. The apparent magnitude of an object varies with the 

 distance of the source of light by which it is rendered visible ; 

 when this recedes, the pencil of light has less divergence, and 

 the object appears smaller ; when, on the other hand, it ap- 

 proaches the eye, the visual cone has a rapid expansion, and 

 the object still held in the same position appears magnified. 



17. All these effects are demonstrable by using artificially- 

 prepared transparent figures, the dimensions of each of which 

 do not exceed the diameter of the pupillary opening of the 

 eye. 



18. It is probable that the minute structure of many natural 

 transparent objects may be recognised in the same way. The 

 hexagonal facets in the eye of the dragon-fly certainly can. 



19. These phenomena are, for obvious reasons, but imper- 

 fectly discriminated by short-sighted persons. 



20. And, finally, it should be noticed that the investigations 

 resulting in the phenomena described in these papers were 

 commenced, and have been conducted throughout, for the 

 specific purpose of testing the power of the naked eye in con- 

 centrating the rapidly-diverging rays of light, proceeding from 

 bodies when held at very short distances from it unaided by a 

 lens ; and from these inquiries it would appear, amongst other 

 results, that the magnifying power of the eye is limited by the 

 magnitude of the visual angle on the one hand, and by the 

 intensity of light on the other. If the visual angle be too large, 

 the rays are not sufficiently refracted by the humours of the 

 eye to converge to a focus, and form an image on the retina ; 

 and if too small, the image is reduced to a mere point. The 

 exact amount of divergence of tlie rays, therefore, for any 

 individual eye lies somewhere between these two extremes. 

 Again, however nicely adjusted the visual angle may be to 

 the refractive powers of the eye, if the light be too strong the 

 pupil becomes so contracted that only the innermost rays are 

 admitted ; while, if it be of small intensity, the object is so 

 dimly illuminated as to be scarcely visible. If, then, whilst 

 a small object is held very near to the eye, so as to insure 

 a rapid divergence of tlie rays proceeding from it, the pupil 

 can be dilated by the small quantity of light which is used, 

 and to which like a photometer it immediately responds, so as 



