258 PHOTOMICROGRAPHY 



required ; the micro-anastigmats are best for low magni- 

 fications. Monochromatic yellow-green light gives the 

 best definition with most objectives, and should be used 

 whenever the colour of the section permits ; the advan- 

 tage of staining with a red dye is therefore obvious. For 

 sections stained with other colours or double stained, a 

 suitable selection of screens, as indicated on Plates 17 

 and 19, is important. 



Fixing the Tissues. It is essential that both plant and 

 animal tissues be preserved in such a manner that as 

 little change as possible should be undergone by the 

 constituent cells. Fixing fluids are used for this purpose, 

 but there is no universal reagent which can be used 

 indiscriminately for both plants and animals ; different 

 tissues require different methods of fixation in order that 

 the best and most faithful results may be obtained. 

 Alcohol will suffice for some of the harder stems and roots 

 of plants, but is liable to cause a shrinkage of the cells of 

 more delicate tissues. For these, particularly where 

 accurate fixation of the protoplasm and nucleus of the 

 cells is required, chromo-acetic fluid is a better reagent 

 (p. 263). For animal tissues a useful general fixative is 

 corrosive-acetic fluid (p. 263), but the solution employed 

 depends greatly on the nature of the tissues to be fixed. 

 The fixing methods are given with the formulae in the 

 Appendix. 



Cutting the Sections. After fixation the objects are 

 ready for cutting and staining. Botanical preparations 

 are usually stained after cutting, but animal tissues are 

 as a rule stained in bulk. 



Stems and roots of plants preserved in alcohol may be 

 cut by hand with a razor, the blade of which is flooded 

 with spirit, small or thin objects such as leaves may be 

 placed between two pieces of elder pith, which act as a 

 support and greatly facilitate cutting. A Cathcart or 

 other simple form of microtome enables one to obtain 



