128 SUMMARY OF CURRENT RESEARCHES RELATING TO 



The segment of a plano-convex lens A has a curvature of • 4 in. 

 radius, and for convenience of mounting, the segment is somewhat 

 longer than is optically necessary. To it is cemented an equilateral 

 prism from which the greater portions of the basal angles have been 

 cut off so as to leave only a small part of the original form of the 

 prism at the apex B. The dimensions of the prism are : — 



Total length 30 in. 



Upper reflecting face "11 j? 



Surface of emergence • 075 „ 



Thickness "1 „ 



Breadth '2 „ 



The lens condenses the rays upon the upper internal face of the 

 prism, whence they are totally reflected and 

 Fig. 22. pass with slight refraction through the lower 



prism-face in the direction of an object placed 

 under the centre of the front lens of the ob- 

 jective. 



Mr, Tolles states that by this arrangement 

 the effective aperture of the object is not 

 reduced, as the apex of the prism is placed 

 just outside the cone of rays which the ob- 

 jective transmits from the object. 

 Such a device seems hardly needed for so low a power as 1 in., 

 but it would be interesting to know if the plan can be successfully 

 used with higher powers. 



Warm and Moist Stages. — Dr. E. L. Maddox describes several 

 forms of these stages which he has devised. 



A slab of ebonite 3^ X 2^ x | in. has a central hole 5-8ths in. 

 in diameter, slightly countersunk on the under side (fig. 23), into 



7 



Fig. 23. 



[11^4^ 



:u— 



which is cemented a circle of stout cover-glass. On the upper side 

 a deep groove is cut at about 1-1 2th in. from the aperture, 1-8 th in. 

 wide, and 3-16ths in. deep; another deep groove concentric with the 

 former being turned out l-12th in. from it, about 3-16ths in. wide 

 and 5-16ths in. deep. Two holes are drilled through one end of 

 the slab, ^ in. apeiTt, ending in the outer groove. At the opposite 

 end of the slab either a deep well is sunk to hold a small circular 

 thermometer, or a hole is drilled through the end of the slab reaching 

 nearly to the outer groove, to hold a clinical thermometer. The 

 opposite holes have two small brass tubes li in. long screwed into 

 them and cemented air-tight. Three screws, furnished with rather 

 wide thin screw nuts, are screwed into the base-plate, one between the 



