THE DRAWING OF MICROSCOPIC OBJECTS. 299 



half-inversion. In practice, the rule also holds good, that the 

 plane of projection in the reflected image must coincide with, or 

 at least be parallel to, the field of view. 



It cannot, therefore, be once for all decided which of these 

 contrivances is the most suitable for convenient manipulation, for, 

 obviously, familiarity and practice may lead different observers 

 to various opinions upon this point. So much, however, will be 

 evident to all, that a half -inversion, such as a single reflexion 

 involves (whereby, for instance, a spiral winding to the right is 

 changed into one winding to the left), must be looked upon as a 

 real disadvantage which not only adds to the difficulty of making 

 the drawing, but which may also, under certain circumstances, give 

 rise to many errors. The reflected image obtained in this manner 

 should therefore be again inverted, which may conveniently be 

 done by tracing it on the reverse side of the paper against a 

 window. 



The method recommended by Harting, to draw the real image 

 produced by the objective on a screen at a distance of 400-500 

 mm., and then make a tracing of it with tracing-paper, as also 

 Stilling's method 1 (which would not be applicable for high amplifi- 

 cations), involve such troublesome minutiae that they are not likely 

 to find many supporters. 



Since the amplification of the projected image is dependent upon 

 the distance of the plane of projection, it must be determined 

 directly for the usual distance and also for different distances 

 (cf. p. 179). This should be done for each objective ; the amplifi- 

 cations should be tabulated and preserved in the microscope-case,, 

 together with the table of the relative values of the micrometer- 

 divisions. 



The scale of the amplification should be given with each figure 

 thus : (60), (350), and so on, or in the form of a fraction, as 

 6 T-> -H 5 -* though the addition of the denominator is superfluous, 



1 Harting ( u Das Mikroskop," 2nd ed. ii. p. 295) describes Stilling's method 

 as follows : " A small piece of tracing-paper is attached with gum upon the 

 cover-glass, under which is the microscopic preparation. This is now brought 

 under the Microscope, and the outlines of the magnified object are drawn upon, 

 the tracing-paper with a fine point. A network of fine lines is afterwards 

 drawn upon it, dividing the whole into small squares ; a similar network of 

 larger squares is made upon a piece of paper, and the drawing on the tracing- 

 paper is copied." 



