23 



culty of obtaining, commercially, lampblack which is free from grit. 

 Nothing is more annoying when cutting a series of sections with the 

 microtome, than to find some of the sections scarred by a gritty par- 

 ticle from the blackened surface carried through on the edge of the 

 knife ; not to speak of the efiect on the knife-edge itself, which rapidly 

 blunts under such treatment. To obviate this the writer tried reversing 

 the block on the blockholder of the microtome so that the painted 

 surface was last cut, but found that the points of the 'ritzer' grooves 

 were then driven back through the soft paraffin, and became in- 

 distinguishable under the microscope. 



To avoid these disadvantages the writer now substitutes for com- 

 mercial lampblack that obtainable by burning camphor. This is ap- 

 plied in two ways. The two methods are alike in that a mixture of 

 absolute alcohol and collodium is used made up in the proportions 

 of 5 c. c. of the former to 1 drop of the latter. They differ as follows : 



Method 1. The grooved side 

 of the block is painted with a 

 brush moistened in the alcohol 

 and collodium and the block held 

 with this surface downwards, for 

 short periods at increasingly longer 

 intervals, over a small piece of 

 burning camphor. This part of 

 the process must be very care- 

 fully carried out so that the par- 

 affin does not melt. The result 

 on the slide is a line almost im- 

 perceptible to the naked eye but 

 excellently adapted for high power 

 projection in drawing for recon- 

 struction with the Zeiss apparatus. 



, ,, 111 Taken with the Zeiss projection and 



In the second method a num- microphotographic apparatus. 



ber of watch glasses, or plates of 



glass or metal, are smoked over the burning camphor. A camel-hair 

 brush is then thoroughly wetted with the alcohol and collodium and 

 as much of the lampblack brushed up from the plates as suffices to 

 put a thin film on the grooved surface of the block when the brush 

 thus prepared is drawn over it. After drying, melted soft paraffin is 

 run over the surface in the usual way and when this has solidified the 

 squaring of the block is completed. 



The directing plane thus produced is illustrated in the acconi- 



