652 Transactions of the Society. 



minant : even two paraffin lamps, each with half an inch wick, may 

 be so used as to give different degrees of intensity of illumination. 

 But each individual would know how to repeat what he regarded 

 as a suitable light. This light, which may be called the standard 

 light (it does not matter what the light is, so that it can be re- 

 produced again at will), is the foundation of the method to be 

 employed. 



In order to explain the use of the table, it must be pointed out 

 that in addition to a satisfactory trial exposure for each kind of 

 subject, there are four other factors controlling the exposure : — 



1. The illuminant used should be so arranged with the trial 

 exposure that it can be repeated at will. 



2. The apian atic cone of light passed by the circular aperture 

 of the stop used beneath the substage condenser to regulate the 

 light for contrast, definition, and resolution, must be known by its 

 N.A. value. 



3. The diametrical magnification on the focusing screen has to 

 be ascertained. 



4. The speed value of the plate. 



5. The nature of the subject allowed for, and a standard trial 

 exposure for each class of subject obtained under the above four 

 conditions. If a screen or light-filter be used, this does not create 

 a new factor, but simply modifies number one, the light with the 

 screen or filter becoming the standard light. 



Now it may be well to call attention to the omission of any 

 reference to the tube-length of the stand, the objective, the ocular, 

 or the camera extension. For purposes of exposure they are only 

 regarded as amplifiers, and as such are considered when the magni- 

 fication of the image is calculated. In practice the objective governs 

 the N.A. of the stop to be used beneath the substage condenser. 

 When this stop or aperture has been selected as the most suitable 

 for the purpose, the light passed by the condenser is limited by it, 

 so that as factors, except as amplifiers regulating the diametrical 

 magnification of the subject to be photographed, the objective and 

 ocular, the tube and camera extension, call for no separate con- 

 sideration. 



It is of the first importance that the N.A. value of the circular 

 stops or apertures used with the substage condenser, or the different 

 openings of the iris diaphragm, should be ascertained and marked 

 for each condenser, and again for using with any portion of that 

 condenser, if it is the desire of the operator to use it with the top 

 lens removed. 



It must not be understood that an ascertained exposure with a 

 particular stop and condenser will apply to a stop of the same 

 value with a condenser of a different focus. This by no means 

 follows. But it would probably be correct with another condenser 

 of the same focus, if the diameter of the back lens were the same. 



