22 



PHOTO-MICROGRAPHY 



purpose of obtaining lantern slides, this careful measurement is of great importance 

 for as the size of the ordinary lantern plate is only about 3 inches square, it is very 

 obvious, if the picture be elongated the entire length of the quarter plate, which is 

 4 inches, a portion of it can never be introduced in the lantern slide at all. To 

 prevent this accident occurring it is a good plan to mark upon the ground glass with 

 a pencil two distinct lines, w^hich show the limit of the lantern slide. 



The measuring being complete and the amount of magnification noted, focusing 

 is now carefully made by moving the object itself by means of the milled head at S 

 with a faint touch to and fro, and not by moving the screen ; for this will be found 

 to be an unsatisfactory method, only capable of explanation by a study of the optical 

 nature of the image in this situation, Avhich would be unsuitable at the present moment. 



We now proceed to substitute for the ground glass the photographic plate 

 (carefully backed) in its slide, and having " turned the lime," in case it takes to 

 flaring at the moment of exposure, which would jeopardise the safety of the ground 

 glass, we set the desired diaphragm and place the cap on the lens. Sometimes this 

 simple procedure is difficult owing to the fact that the lens is not more than two 

 thicknesses of its cap from the object. When this so happens a black card passed 

 between the lens and the object, resting on the foot of the apparatus just above the 

 screw shown in the diagram, will quite sufficiently cut off" the light from the inside of 

 the camera. Having drawn the slide it is well to wait for a few seconds to allow the 

 "shake" in the whole apparatus to settle down. 



Exposure will, of course, vary according to the diaphragm employed in the lens. 

 F/16 to F/22 are about the limits. At the former aperture using the Planar lenses 

 we have found the range of exposure is 2 to 5 seconds if the object be an uncoloured 

 one, but much more if it be coloured. 



It has been mentioned that the distance of the lens from the object and the 

 camera length are factors for producing the required magnification. So they are, but 

 it must not be thought that the focal length of the lens itself has been lost sight of. 

 What has been said applies in a fashion to the use of the sam.e lens. Seeing, 

 however, it may be of service to the photographer to rightly understand w^here the 

 focal length of any lens he may use comes in, we will conclude this section of our 

 subject by giving a few formulae, which, although they may not be rigidly correct, are 

 quite correct enough for all practical purposes. We shall ignore the "covering 

 power " here as it has been spoken of before, and confine ourselves to answering three 

 questions : 



