644 Transactions of the Society. 



circle. It is thus certain that the prisni will pass the full cone of 

 any ordinary high or low power objective. 



With the 24 mm. apochromat of N. A. • 32, and a large axial 

 illuminating cone, the minute spines on the membrane of the blow- 

 fly's proboscis appeared jet black and remarkably clearly defined, 

 the focus not requiring the slightest alteration when the ocular was 

 employed with and without the prism. The fine quality of the 

 image also appeared absolutely unimpaired by the addition of the 

 prism. 



With the 12 mm. apochromat and a full solid illuminating 

 cone on an extremely sensitive object, the tube-length correction 

 for thickness of cover-glass was found to be precisely identical for 

 the same ocular when used alone or with the prism added, the 



c 



A B 



Fig. 107. 



A. Back view of prism. B. Side view. C. Ramsden's disk. 



The dotted lines show the paths of the marginal rays. 



colour correction also proving most satisfactory when tested on the 

 N. lyra. 



The annexed figure (fig. 107) shows the form of the prism. The 

 lower surface, that next the eye-lens of the ocular, is convex to 

 allow the rays to enter the prism normally. The upper surface is 

 concave in order that these rays may be discharged into the air 

 without refraction. These curved surfaces form a meniscus lens, but 

 by selecting suitable curvatures with respect to the path of the 

 rays through the prism, the lens is made so long in focus that it 

 has no effect upon the Microscope image. 



Should a prism be required for use with only one particular 

 eye-piece, 'it would be doubtless preferable to combine it with the 

 eye-lens of the ocular, making the eye-lens and prism of one piece 

 of glass by adjusting the lower surface B to the proper radius. 

 But an appliance which can be utilised at need on a variety of 



