64 PHOTOMICROGRAPHY 



The image of a point formed by a lens is a disc of very 

 small size. The image is in focus when that disc or circle 

 of confusion is less than ^gV' in diameter, in which 

 case it appears as a point to a normal eye at 10" 

 distance. The condition under which the images 

 formed in one plane of two points in different planes 

 shall appear sharp, is that they are both smaller than 

 the greatest permissible circle of confusion. If one point 

 is in focus the size of the image of the other depends not 

 only on its vertical distance from the first, but on the 

 breadth of the cone of light producing the image, that is, on 

 the N.A. of the objective, and, of two lenses of the same 

 power, that with the lower N.A. will have the greater 

 penetration. The best means of obtaining greater 

 depth of focus, without altering magnification, is to 

 substitute a lens of lower N.A. of the same power, or a 

 lower-power lens combined with greater camera exten- 

 sion. There is in either case a loss of resolution, but 

 resolution, as a rule, is not required at the same time as 

 penetration. 



Another method for increasing penetration, and with- 

 out a change of objective, is to reduce its aperture by a 

 diaphragm above it, or by restricting the aperture of the 

 condenser ; as the iris is closed points further and further 

 away in vertical direction come progressively into 

 sufficiently accurate focus. But there is a limit to this 

 use of the diaphragm, as diffraction effects are also pro- 

 duced (p. 121). 



An increase of the refractive index of the medium in 

 which an object is mounted helps the objective to simul- 

 taneously focus more than one plane. As an illustration 

 of this the well-known experiment, whereby a stone 

 invisible in an empty jug becomes visible when water is 

 poured in, may be cited. In a similar way, supposing the 

 bottom layer of an object mounted dry is slightly out of 

 focus when its top is focussed, it may be brought into 



