TRANSACTIONS OF WAGNER 

 54 



MENDEL AND HIS LAW 



conveniences of holding parts of the instrument have slowly changed its de- 

 tailed form, but very little fundamental difference has as yet crept in. The 

 higher the power of a microscope, the greater the curvature of its front lens, and 

 consequently the smaller its size, so that when we come to need magnifications 

 of a thousand times or more, the front lens is very small and admits but little 

 Kght. Some of this little Ught is lost as it passes out of the object through the 

 air into the lens. Modern practice fills this gap when higher powers are used 

 with a drop of an oil whose refractive effect on Hght is the same as that of glass 

 itself, and this avoids the loss which comes of breaking into the air and back into 

 glass. Such a combination is called an oil-immersion lens. 



To these refinements has been added in recent years the practice of cutting 

 the tissue to be examined into exceedingly thin slices. In order to do this, the 

 material to be examined is first saturated with melted paraffin and then allowed 

 to cool in a block of paraffin. This block can be fastened in the clamp of an 

 instrument called a microtome. There a razor approaches the block (or the 

 block approaches the razor) in such way as to slice a wafer of paraffin from the 

 end of the block. The turning of a screw raises the block of paraffin and an- 

 other stroke gives a new slice of paraffin with the tissue embedded in it. The 

 fineness of the screw and the extent to which it is turned determine the thick- 

 ness of the successive sHces. There is no difficulty in cutting slices thinner 

 than ToVo of an inch. The necessity for this lies in the fact that a microscope 

 portrays a picture which has length and breadth but practically no thickness. 

 That is to say, there is no depth to the picture. Everything must be on one 

 level or escape plain observation, and unless it is very near the level, it escapes 

 observation entirely. Hence the necessity for thin sHces, and the higher the 

 power of the microscope used, the thinner it is necessary these sHces should be. 



The study of chick embryos in Balfour's laboratory in Cambridge, Eng- 

 land, led to the introduction of an interesting modification of section cutting. 

 It was found that if the paraffin block was of the right consistency and cut 

 perfectly square, each sKce adhered by its edge to the edge of the next slice. 

 Thus a ribbon of paraffin was produced in which the sHces followed each other 

 in regular succession. This ribbon could be cut into short strips, fastened 

 upon the microscope sHde, the paraffin dissolved out, and a cover-glass cemented 

 over the strips so as to keep them permanently. It was possible in this way 

 to embed a small animal, cut it into serial sections, which could be pasted in 

 order on a succession of sHdes in such fashion that, studying one section after 



