102 HIGH POWEK PHOTOMICROGRAPHY 



In order to surmount the difficulty of imperfect achromatisation ol 

 the lenses, a green hlter is used and photographs are taken on process 

 plates sensitive to this light. In all apochromatic lenses there is 

 always a good deal of curvature of field, and we should like to suggest 

 that for photomicrographic purposes it would be useful to have a lens 

 without any colour correction, if the elimination of other forms of 

 aberration and curvature of field would be thereby facilitated. 



The illumination used is always what is usually termed critical, 

 fehat is to say, the light source is in focus on the plate at the same time 

 as the object being photographed, this being rendered possible by the 

 uniformity of illumination over the whole of the hght source. In this 

 connection we should like to put forward a theory of the well-known 

 phenomenon of the flooding of light over the image at critical illumina- 

 tion when the aperture of the condenser is fully open. \Ye believe that 

 the explanation of at any rate a part of this is that the miage of the 

 light source which lies in the plane of the object is not an infinitely 

 thin plane, and there is so little depth of focus with a high power 

 objective that we have the effect of the image of a bright surface 

 lying just in front or just behind the object and out of focus on the 

 plate, producing the so-called flooding effect. If we cut down the 

 aperture of the condenser we eventually use only light which is almost 

 parallel, and therefore obtain a shadow photograph which is absolutely 

 free from flooding. If we cut down the aperture only a small amount 

 we may do so sufficiently to make the effect of flooding negligible. In 

 support of this we may mention that flooding is not obtained if the 

 image of the light source is very much out of focus. In the " Pointo- 

 lite " lamp the curvature of the light source will contribute to this 

 effect. 



In some of our earlier work we used an achromatic lens between 

 the " Pointolite " lamp and the condenser, but it was the light source 

 which was always brought to a focus on the screen, and not the image 

 of a diaphragm over the lens, as is sometimes done. This lens was 

 used to magnify the image of the light source so that a larger part of 

 the object could be illuminated, but the same effect is now secured by 

 bringing the lamp as close as possible to the condenser. In this way 

 we can illuminate an area of the object, which is a little larger than the 

 flat part of the microscopic field. This increases the ease of aligning 

 the optical system, and moreover slightly increases the working dis- 

 tance of the condenser, which, however, is never much more than 



1 mm. 



The exposure with the orthochromatic process plates in use, with 

 the green filter and a magnification up to 2,000 diameters, varies from 



2 to 10 seconds. In this connection it is important to note that for 

 all work requiring the greatest resolution process plates (i.e., plates 

 with a hard working emulsion*) must be used. (Goldberg, P. J., 52, 

 302 (1912).) 



Laboratory of the British Photographic Research Association, 

 Chemical Department, 



University College, 



Gower Street. 



•■ In the December number of the Photographic Journal we have 

 shown what type of emulsion is required to make a good process plate. 



