OX CONSTRUCTING STANDS OK MICROSCOPES. 103 



tions for traversing, and indeed I prefer it myself, (pro- 

 vided the object does not yield too easily,) even with very 

 high powers. 



Lastly, it has been remarked, with great justice, that 

 if we choose to employ any of the generation of camera 

 lucidas for drawing objects, they can only be used with 

 microscopes in which the body is altogether immoveable. 

 Nothing is more certain. At the same time I cannot re- 

 frain from expressing my hatred and contempt of the 

 Whole! tribe, as rather impediments than helps to a real 

 draughtsman. This much I will venture to say, that if 

 a man cannot draw ivithout them, he cannot ivith them. 



The slider-holder enables the present construction to 

 operate with these nuisances, for the adjustment of the 

 focus will not sensibly disturb that of the image on the 

 paper, and if it should, a very little motion, backwards 

 or forwards, given to the drawing, will re-arrange it. 

 The only aid which I can with confidence recommend*, 

 as of real utility, is a micrometer, composed of glass, 

 placed in the field bar of the microscope, and divided 

 into about 40 or 50 parts to the inch, with double divi- 

 sions, or very strong lines at every fifth space. These 

 should be rubbed over with black lead, after which they 

 must be burnished with the edge of a piece of hard 

 wood, so as to clean the surface of the glass ivithout rubbing 

 the black lead out of the divisions, (a device of the late 

 Dr. Wollaston, and bearing the stamp of his original and 



* See Micrograpbia, p. 221. 



