Exposure luith Transmitted Light. By A. Letherhj. 653 



It may afford some assistance if it is shown how tliree con- 

 densers with which the same fitting and stops are used are 

 tabulated. These stops or apertm-es are numbered,* and against 

 each stop is recorded its ascertained N.A. value with each con- 

 denser. 



Powell and Lbaland Oil Immebsion Condenser N.A. 1-4. 



Stop 



Top lens r 

 5 used dry 

 G 

 7 



emoved. 



9 

 10 



N.A. 0-4 



0-3 



0-25 



0-2 



015 



0-12 



Powell and Lealand Apochromatic Condenser N.A. 0'98, 



Stop 10 used dry 

 9 



8 



6 

 5 

 4 

 3 



Beck Oil Imm 



Stop 3 used with oil 



4 



As water imm. with full aplanatic aperture 



Condenser N. 



N.A. 0-25 



„ 0-3 



„ 0-4 



„ 0-47 



„ 0-55 



„ 0-65 



„ 0-85 



„ 0-95 



A. 1-4. 



N.A. 1-25 

 „ 0-9 

 „ 1-1 



If the table is consulted, it is seen to be composed of columns of 

 figures. The first column represents the aperture used to limit the 

 light passing through the substage condenser. The other columns 

 show what magnifications of the object to be photographed can be 

 used with the same exposure, but each different magnification in 

 the column needing the aplanatic aperture or stop in a line with 

 it, shown in the first column. At the top of each column is the 

 relative time of exposure for all the figures in that column as com- 

 pared with the time of exposure needed by the magnifications in 

 all the other columns. 



Note that the fifth column of figures under time 1, commences 

 with X 400 on a line with N.A. 1 • 0, and terminates with x 20 on a 

 line with N.A. 0"05, and each intermediate x has its con^esponding 

 N.A. Any of the figures in the column used with the N.A. on the 

 same line (or corresponding aperture) requires the same exposure. 



The application of the table will be more clearly understood, 

 and the principles best and most easily explained, by giving two 

 or three practical examples. 



Set up the standard light, which may be of any kind pro- 

 vided it can be repeated at will, then obtain the diametrical 

 magnification of the object to be photographed in the usual way, 



* Not Powell and Lealand' s numbers. 



