Wenham, on a Binocular and Single Microscope. 19 



should be of a width only just requisite to include the 

 half aperture of any object-glass, one quarter of an inch 

 is quite sufficient ; it 



should not exceed this ^'g- 2- 



for two reasons, first, that 

 the greater the thickness 

 of glass that the ray has 

 to pass through, the more 

 difterence there will be in 

 the magnifying power of 

 the two bodies, and second, 

 that a thick prism takes 

 the ray more away from 

 the centre of the main 

 tube, and increases the 

 convergence of the two, 

 bringing the eyes nearly 

 approaching to the dis- 

 agreeable condition of a 

 squint. 



Both the transmitting and reflecting surfaces of the prism 

 should be accessible for the purpose of wiping, for any par- 

 ticles or mildew adhering to the latter will prevent total 

 reflection at the point of contact. If the prism is well made 

 and polished, and of the smallest size possible for admitting 

 the pencil, the difference between the direct and reflected 

 image is scarcely appreciable, and with this standard of com- 

 parison a faulty prism will immediately be detected. By 

 pressing back the spring catch or stop on one side of the 

 prism-slide, it can instantly be withdrawn altogether, and as 

 quickly replaced. 



