ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



371 



Table of Exposures for Medium and High Magnifications. 



•Using edge of f-in. flame of Kerosene lamp, Nelson paralleliser, and condenser 



at its critical focus. 



For llford Chromatic Plates. 



Note. — The higher exposures are for fully exposed negatives. Nemst 

 1-ampere filament lamps require J;, of these exposures. 



A table of exposures for low magnifications, at low working- apertures, 

 based on the same principles, is also given. The following practical 

 points in connection with making the measurement of the Ramsden disk 

 are mentioned : (1) It is easier to make it with a low-powered eye-piece, 

 which can then be changed for the one with which the photograph is to 

 be taken ; (2) it is generally better to move the object to be photo- 

 graphed to one side, whilst it is being done : (3) it is necessary to 

 interpose a screen of smoked or dark green glass between the eye and 

 the light, which should then be removed before the exposure is made ; 

 (4) the markings on the millimetre measure should be coarse. Anyone 

 who intends to use this method of estimating exposures should work 

 out a standard for himself, as the intensity of the illumination depends 

 so much on the arrangement of the collecting and condensing lenses, 

 and on the source of light. If found to be the same as in the table, 

 well and good ; if not, a magnifying or dividing factor could be found 

 to be used with this table, or, if preferred, a new table could be worked 

 out. If yellow or other coloured screens are employed, they ought to 



2 c 2 



