194 PEACTICAL PHOTO-MICKOGEAPHY 



from its upper principal focal plane. It is sufficiently accurate 

 for all ordinary purposes to measure this distance from the 

 upper lens of the objective, so that an image projected into 

 the camera at a distance of ten inches from that point would 

 give a magnification of ten times. If the camera-length is 

 increased to double the above dimension, the magnification 

 expressed in diameters is then doubled, and therefore becomes 

 twenty times, and so on, for any increase of distance. We 

 therefore get for this distance of ten inches the following as 

 the initial magnifications to be obtained with lenses of the 

 indicated focal lengths : 



A lens of 1 inch focus or 24 mm. = x 10 

 t ?? 12 = X 20 



1 6 = X 40 



i Q \/ QH 



8 X OU 



i- 2 ,, 2 = x 120 



These are all on the assumption that the focal length of the 

 lens is exactly that indicated ; but it rarely happens that on 

 actual test this is found to be the case. 



In photo-micrographic work there are three factors that 

 can influence magnification : the power of the objective, the 

 power of the ocular, and the length of the camera. It is 

 therefore possible to obtain a certain magnification with 

 different objectives that is, within limits and the method 

 that may be adopted to obtain any given magnification must 

 always be determined with reference to the object itself. 



It has already been pointed out that the quality of pene- 

 tration, for instance, depends on the N.A. of the objective, 

 and in general this N.A. is dependent on the focal length of 

 the lens. In cases, therefore, where an object is of considerable 

 thickness, and where, it is necessary to obtain the best 

 compromise possible of varying planes in it, it would be advis- 

 able to use a lens of the greatest focal length, and to obtain the 

 necessary magnification either by using a higher power ocular, 

 or by increasing the camera-length. No definite rules can be 

 laid down in this matter, and it is only by careful considera- 

 tion in relation to the object itself and after perhaps a fair 

 amount of experience, that the best combination can be 

 exactly determined. 



