ro4 



SUMMARY OF CURRENT ■ RESEARCHES RELATING TO 



tive / throws on the slit a real image of the light source. With sun- 

 light the distance of the slit from the objective is about 80 mm., and 

 the sun's image is then 1 mm. in diameter. The slit-head is shown 

 separately in fig. 108. The drum c, graduated on rim into 50 divisions, 

 must always be on top : the entrance-slit is then horizontal. One 

 complete rotation of the drum opens the slit about | mm., so that the 

 rotation of one division of the drum produces an aperture of T ^o mm. 

 For the examination of fluids the most favourable slit-breadth is from 

 O'l to 0*4 mm. The figure also shows the two slit-cheeks, one of 

 which is secured by springs and is operated by the screw a. These 

 cheeks limit the slit-length, which, with sunlight, should be about 



Fig. 108. 



1 mm. The whole head can be rotated through 90° by the lever I). 

 The effect is to make the slit vertical, which is of importance for 

 gauging purposes. The slit answers a two-fold purpose : firstly, a 

 measurable, illuminated volume is delimited in the preparation ; secondly, 

 the depth of this volume must be made as suitable as possible to the 

 penetrating power of the Microscope objective used (in this case, the 

 water-immersion D*). 



Details of the gauging process are given below. At h (fig. 107) 

 there is a second projection objective of 55 mm. focus at a distance of 

 about 14 cm. from the slit. Like the first objective, it is mounted on 

 a rider with its front side directed towards the Microscope. It casts a 

 real, about 1J fold reduced, image of the slit at a distance of 90 mm. 

 from the lens. The function of this second lens is to bring the image 



