396 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



means of substage focusing, the lens to be measured is racked up until 

 the image of the card is sharply focused. Then one of the sides of 

 the card is spanned by the webs of the eye-piece micrometer, and its 



q 



M 



I*- 



pi cj 



jpp. inches u 



Mr 



2" 



Fig. 34. — Diageam to show Relative Positions of the Appabatus. 



M. — Microscope tube. B. — Objective. 



A. — Screw micrometer. C. — Objective to be measured, in substage. 

 S. — Microscope stage and micrometer. 



size measured and the magnifying (or, rather, diminishing) power found ; 

 then 



100 



f 



m + 2 



Of course, the idea of the 5 in. is that the reading is doubled, and then 

 10 -r x (say) gives the magnification, m, which can be found from 

 reciprocal tables, as well as the value of 



100 



m + 2 



It is not difficult, but a little more trouble, to make the calculations 

 without tables. 



This method will also measure the foci of large photographic lenses. 

 In that case 



, 100 100 



m 



+ 2 (m + l) 3 



This second term is only necessary when / is large compared with 

 100 in. ; for microscopic lenses it is not wanted. 



The screw-micrometer eye-piece is perhaps a drawback, as an ordinary 

 screw-micrometer with a negative eye-piece is no good for lens measure- 

 ments. The eye-piece must be of the Ramsden type, and it is very 

 doubtful if any ordinary ruled glass micrometer eye-piece would be 

 sufficiently accurate. A screw-micrometer is necessary for both the 

 methods described. 



(3) Illuminating and other Apparatus. 



Improved Form of Cheshire's Apertometer.* — J. Grundy describes 

 E. ]YI . Nelson's improved form of Cheshire's valuable instrument, the 



* Journ. Quekett Micr. Club, xii. (1914) pp. 281-2 (1 fig.). 



