374 PHYSIOLOGICAL PHYSICS. [Chap. xxix. 



be capable of considerable extension, and on extending 

 it an image of the object will be cast on the ground- 

 glass plate. One initial obstacle in the way of micro- 

 photography is the great loss of light that is involved 

 in the arrangement, and the consequent difficulty in 

 the way of using high powers. To some extent this 

 is overcome by the extreme degree of sensitive- 

 ness which can now be given to photographic plates. 

 Any one can now attempt micro-photography without 

 going through a long apprenticeship in the preparing 

 of plates fit for working with. Plates of extreme 

 sensibility can be readily procured, and all one has 

 to acquire is the art of taking and developing the 

 picture, since material to work with of the best pos- 

 sible description is to be had at comparatively small 

 cost. 



CHAPTER XXIX. 



THE EYE AS AN OPTICAL INSTRUMENT. 



Camera obscura. We saw (page 300) that if 

 a small opening exists in the wall of a dark chamber, 

 the rays of light from the outside passing through the 

 opening will form an inverted image of the external 

 object on the opposite wall of the chamber. Unless 

 the opening be very small, the image will be blurred 

 and indistinct from the overlapping of rays from 

 various points of the object. If the opening be small 

 enough the overlapping rays are cut off, and a distinct 

 image formed, but a very dim one, owing to the loss of 

 light. If, however, a conyex lens be interposed in the 

 path of the rays, the opening may be enlarged, and the 

 various rays are brought to a focus so that the images of 

 diffusion are prevented. Now the dark chamber or 



