MAGNIFYING POWER OF SHORT SPACES. 13 



substances are most brilliant when viewed in light of their 

 own colour. 



By way of recapitulation, I shall beg to sum up with the 

 following remarks : — 



1. When small bodies are brought very near to the eye, 

 their images are magnified, just as images of larger objects 

 when seen at a distance are diminished, and by the same law. 



2. The apparent magnitude of objects depends on their 

 visual angle. 



3. Tlie visual angle, for short distances, may be well illus- 

 trated by employing a small circular disc of light. 



4. A minute circular disc of light is procured by perforating 

 a card with a needle, through which the light is then permitted 

 to pass. 



5. A sewing-needle, of the size marked No. 6, produces an 

 aperture about the one-fortieth of an inch in diameter. 



6. In order to examine the light which is transmitted 

 through such an aperture, all extraneous rays should be ex- 

 cluded ; hence the plane in which the opening is made should 

 be placed at the end of a tube. 



7. The pencil of light admitted through an opening of this 

 kind, held within an inch or so of the eye, consists of rapidly- 

 diverging rays falling upon the cornea. Some of these are 

 entirely lost, others are intercepted by the iris, while the re- 

 mainder pass on through the pupil, which communicates to 

 the image formed on the retina its circular form. 



8. Whether the small aperture itself be round or triangular, 

 square or irregular, in form, provided its area do not much 

 exceed that of a circle the one-fortieth of an inch in diameter, 

 its image is always circular, 



9. When more than one aperture is used, and these of 

 different tints, secondary colours result from the overlapping 

 and blending of the images of the primary. 



10. If the three primary colours, yellow, red, and blue are 

 used, their images, which overlap in pairs, produce orange, 

 violet, and green light ; and when the images of all three 

 blend white light is the result. 



11. When a small transparent object is held close to the 

 eye it is altogether invisible. 



12. But its outline is immediately determined by the light 

 transmitted through one of the small inlets above described, 

 and it is then seen to be not only magnified but inverted. 



13. The image becomes much more distinct when more 

 than one aperture is used, for the intensity of light by which 

 it is illuminated is thereby increased, being almost in a direct 

 ratio with the number of the openings which are employed. 



