Exposure with Transmitted Light. By A. Letlierhy. 655 



and the plate used must be of kuown speed in order to make the- 

 needful calculation. The subject must also be allowed for in order 

 to get the time value of the trial exposure. 



Example 1. — Suppose the object is a diatom mounted in 

 styrax. It is placed under a \ in. objective x 40, with a tube 

 extension of 10 in., and a x 5 ocular. The camera extension is 

 10 in. We have now an image X 200. An aperture stop beneath 

 the condenser previously ascertained to have a light value of 

 N, A. • 50 is placed in the usual position. Now a trial exposure, 

 say with a circular wick parafhn lamp and bullseye 10 in. from- 

 the mirror, is found to give, on a medium ortho plate, a satisfactory 

 result with three minutes' exposure. This now becomes a standard 

 time with the same light and plate and an object of similar colour 

 and density. The magnification and stop beneath the condenser, 

 the objective and the eye-piece, the tube-length and the camera 

 extension may all vary from that used in securing this trial 

 exposure, but from it the time needed for any other exposure with 

 the aid of the table can be at once ascertained, provided the con- 

 ditions and factors before refeiTed to be conformed with. 



Example 2. — Now suppose the diatom is placed under a x's'ii^- 

 objective and the N.A. value of the aperture used with the sub- 

 stage condenser is N. A. 1 ■ 0, and the amplification produced by 

 objective, ocular and camera extension is x 400, the exposure with 

 the same light, plate, and condenser is still 3 minutes. 



Example 3. — Now use a more powerful ocular, and let the camera 

 extension be greater so that the amplification becomes x 2000 : we 

 then find from the table that the x 2000 with N.A. 1 • requires 

 24 times more exposure than column marked time 1, that is, 

 72 minutes. Of course, the original exposure wdth a more power- 

 ful light might only require 1 second instead of 3 minutes ; in that 

 case the time would be 24 seconds instead of 72 minutes. 



Example 4. — The camera and Microscope have been set up again 

 with diatoms in styrax as before, and it is desired to photograph in^ 

 order to obtain depth of focus rather than extreme resolution. The 

 measurement shows the magnification to be x 200. The N.A. 

 value of the aperture used with the condenser is 0*25. A reference 

 to the table shows that four times the trial exposure is needed, that * 

 is, 12 minutes. 



For oblique light the trial exposure would probably serve also, 

 if circular apertures were used in the periphery of the condenser 

 and the light derived from a large solid cone, and provided the 

 whole of the light value of such aperture were passed by the 

 objective. Thus a stop having, if used in the centre of the con- 

 denser, a value of N.A. 0'25, or N.A. 0*40, might be used in the 

 periphery of a condenser capable of giving an aplanatic cone of 

 N.A. 1 • 3. The table for aperture value of N.A. * 4 only provides 

 for a magnification of x 900, but it can be extended in every 

 direction to meet the needs of any emergency. 



