ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 517 



depends upon the relation between w and w'. If w is too great in 

 relation to w' it must be reduced. 



It is generally assumed that to ensure the best conditions the 

 paper and the field of view must be equally illuminated, i. e. to = w' ; 

 whilst in fact w' should be greater than w. This may be proved by 

 obscuring half the field of view by a semicircular piece of cardboard 

 placed upon the diaphragm of the eye-piece. Using a weak objective, 

 and having diminished the illumination until it is most convenient 

 for drawing the object with the camera, shift the drawing paper until 

 it occupies only the obscured half of the field ; it will then be seen 

 at once that the field is much darker than the paper, i. e. w is less 

 than w'. 



On the other hand, the brightness of the paper must not be too 

 great in comparison with that of the field, or the object will not be 

 clearly visible. In the use of high powers, therefore, the illumination 

 of the paper must be reduced by interposing glass of difierent tints 

 between the camera and the paper. These should, however, be 

 sparingly used, and only when the illumination of the paper is such 

 as to obscure the object. 



Vorce's Combined Focusing and Safety Stage for use in Micro- 

 metry with High Powers.* — Mr. C. M. Vorce's device (fig. 102) 

 consists of two perforated brass plates, the upper bearing two spring 



Fig. 102. 



clips to hold the slide, and the lower having springs lifting the upper 

 plate, and also a micrometer screw at each end passing up freely 

 through the upper plate, which is depressed by milled nuts on the 

 micrometer screws, opposed by the lifting-springs of the lower plate. 

 " The object of the device is to move the slide instead of the objective 

 in focusing, in order that when making measurements by projecting 

 the image on a screen the distance of the screen from the focal point 

 of the objective may remain absolutely unchanged, which is necessary 

 to avoid the objection that the power has been changed by changing 

 this distance. In micrometry it is essential to avoid, so far as 

 possible, every theoretical as well as every practical source of error, 

 even if it should be too minute to efiect, appreciably, the result. And 

 especially is this true of micrometry applied to determine, judicially, 

 important questions. In micrometry there are, with all the ordinary 

 appliances, some theoretical sources of error, which, although in most 

 cases so minute as to be, in their effect upon the accuracy of the result, 

 practically nil, are sufficient to afford pretexts for objection on the part 



* Proc. Amer. Soc. Micr., 8th Ann. Meeting, 1885, pp. 115-9 (3 figs.). 



