24 PKACTICAL PHOTO-MICROGRAPHY 



exactly in the focus of the objective, as would be obtained by 

 utilising a higher numerical aperture. 



The evil of carrying the reduction of aperture too far is that 

 diffraction fringes very soon begin to appear around the object, 

 and should these diffraction fringes be sufficiently evident and 

 large enough to overlap, then a general want of definition 

 results. 



Another factor influencing penetration is the density of the 

 medium in which the object may be mounted; a more highly 

 refracting medium in which an object is mounted giving an 

 appearance of somewhat greater penetration than one of less 

 refractivity. It follows that an object mounted in Canada 

 balsam would be better suited, on general grounds, for photo- 

 graphic purposes than one mounted dry. Diatoms, or similar 

 objects, which are mounted in media of high refractive index 

 such as realgar or styrax might be expected to give some- 

 what better results in this direction, other things being equal, 

 than would be the case if they were mounted in less dense media. 



Penetration decreases directly as the N.A. of the objective 

 is increased ; but what is more important in photo-micro- 

 graphy, it decreases as the square of the focal length. For 

 example, a -^-inch objective of 0-5 N.A. has four times the 

 penetration of a ^-inch of the same N.A., whereas if the 

 N.A. were halved, the focal length remaining the same, the 

 penetration would only be halved also. 



Hence, for photography, the principle is a sound one 

 always to use an objective of the lowest power and the 

 highest possible N.A. that will show the desired detail. 



Metallurgical specimens when illuminated by reflected light 

 are mounted without a cover-glass, and therefore without any 

 mounting medium. This is to obviate the extra reflection that 

 would occur from the surface of the cover-glass ; and with 

 such preparations better results are obtained by the use of oil- 

 immersion objectives than with dry ones. The immersion fluid 

 is in this case placed directly in contact with the specimen, 

 and so the conditions again occur in which the greatest pene- 

 tration is obtained. For the same reason in very low-power 

 photo-micrographic work, where a magnification of only a few 

 diameters is desired, or where even a reproduction in natural size 

 is wanted such as in photographing bacteriological culture-tubes 



