118 THE MICROSCOPE. 



this a special arrangement is needed for the prism, -which 

 must be set close behind the lens of the th or 

 T Vth inch, in order to obtain an entire field of view in 

 each eye. This it is found to accomplish perfectly when 

 placed in that position ; but still, for very delicate test- 

 objects, requiring the utmost extent of aperture for their 

 definition, it will not resolve them as clearly as with a 

 single body, from the fact that the aperture is divided 

 and half only effective in each eye. This difficulty has 

 at length been nearly overcome by Messrs. Powell and 

 Lealand, by means of an inclined disc of glass with 

 parallel sides, the partial reflection from the under surface 

 of which is again reflected into the second eye, by means 

 of a rectangular prism. As the disc is made thinner, so 

 do the images approximate and the distance between them 

 diminish. Therefore, if the glass is made as thin as pos- 

 sible, and a very slight angle givea to the two sides, it 

 may be so arranged that both images are ultimately com- 

 bined at the eye-piece. There would be no difficulty in 

 working the glass to a mean thickness of 5^5 th of an 

 inch. In this form the angle between the sides would be 

 so exceedingly small that - the chromatic effect, considered 

 as a prism, would be inappreciable in the direct eye-tube. 1 

 " A strong light should be avoided for the illumination 

 of objects observed with the binocular microscope, as 

 direct rays tend to destroy the stereoscopic effect. The 

 illuminator that has been found to give an excellent effect 

 consists of three plano-convex lenses, so combined as to 

 give a very large area of light, as well as great intensity. 

 The final emergent pencil should have, if possible, an 

 angular aperture of 170. Just above the top lens should 

 be .placed a sliding-cap, the crown of which is covered 

 with a diffusing film. For this, the best material is the 

 beautiful snow-white powder obtained from turning glass 

 with a diamond turning-tool. This may be obtained from 

 the opticians, and should be well washed to free it from 

 the larger particles. A thin film of this impalpable 

 powder should be compressed between two discs of thin 

 glass, and fixed in the top of the sliding-cap, which is to 

 be raised or lowered till the most intense light is obtained 



(1) Quarterly Journal of Mwroscopieal Science, vol. i. 1861, and vol. vi. 1866 



