165 



back, so that the eye may be applied to the eye- 

 piece. It is composed of two rectangular prisms, 

 cemented together at their diagonal surfaces, but 

 having between them a perforated film of silver. 



Fig. 44. 



At one side of the prism is a large mirror which 

 reflects the image of the paper and pencil onto the 

 silvered surface of the cemented prism and this 

 in turn to the marginal portion of the pupil, 

 while the axial portion of the eye views the image 

 through the perforation of the silvered surface, 

 thus giving both views coincidentally. 



A number of precautions are to be taken to 

 obtain good results. First of all it must be 

 remembered that if spectacles are necessary in 



