240 PHOTOMICROGRAPHY 



cases in which it is possible to remove small portions of 

 a growth while under treatment ; for example, inoperable 

 cancer under Radium treatment, and rodent ulcer under 

 X-rays, Ionic medication, or Carbon dioxide snow. 

 Plates 31 and 32 are from such a series and show the 

 change brought about in cancer by Radium treatment. 

 Such photographs are more convenient for reference than 

 the original slides, and quite satisfactory if typical por- 

 tions of the section are picked out. 



Photographic records may also be kept of conditions of 

 which other evidence cannot be conveniently preserved ; 

 casts occurring in urine, and the changes in the blood 

 corpuscles which occur in cases of the progressive 

 leukaemia, may be cited as examples. 



Photomicrographs of sections should be taken at both 

 low and medium magnifications to fully demonstrate the 

 general relationships of the tissues, and the more minute 

 structure of their component cells. Powers of 1 \" and f " 

 (36 mm. and 16 mm.) giving magnifications of 50 to 100 

 diameters, and J" or J" (6 mm. and 4 mm.) objectives 

 giving 300 to 700 times, are generally suitable. The J" is 

 advisedly of moderate N.A., about 0-7 rather than 0-9, 

 as the increased depth of focus is frequently of value. 

 A green screen is the most generally useful for contrast, 

 but an orange filter may prove more satisfactory with 

 blue sections. It is weE. to employ a cooling trough to 

 preserve the sections from the concentrated heat of an 

 intense illuminant. 



The photography of large specimens is described later 

 under the heading " Solid Objects," and the bacteriologi- 

 cal methods, which are rather specialised, are dealt with 

 in the next section7 



Stained Preparations of Bacteria (Plates 33 and 34). 

 Diffraction effects, usually noticeable as white lines 

 round the organisms, lack of contrast, and hazy images, 

 are the faults most generally encountered in photo- 



