GENEKAL PEELIMINAKY PKEPAKATIONS 191 



The movement of the image is perhaps only what might be 

 anticipated ; for the weight of the moving parts is such that 

 there is considerable thrust, and it is only natural that this 

 should become evident when the mechanism is actuated and 

 the direction of movement reversed. 



In the greater part of high-power work it will be found that 

 the plain-glass screen, used in conjunction with a focussing 

 eye-piece, will be the most satisfactory ; but it often happens in 

 bacteriological or similar preparations that it is easier to do 

 the fine focussing on the ground -glass screen. It must be 

 remembered that these minute bodies are themselves either 

 cylindrical or spherical, so that there is no absolutely sharp 

 edge on which to focus. It therefore often follows that a better 

 idea of the actual image is conveyed when it is seen on a screen 

 that gives a concrete image. 



It may happen with high-power illuminants that the image 

 is too brilliant to focus with safety on the plain-glass screen 

 by means of a focussing-ocular. In this case on no account 

 must the sub-stage condenser-diaphragm be contracted ; apart 

 from the fact that the optical conditions are being interfered 

 with, there is always the risk of some slight de-centration 

 of the sub-stage condenser, if the diaphragm is interfered 

 with at such a late stage of the operation. If there is too 

 much glare, then the light must be modified by placing 

 some bluish-grey glass, or a piece of the colour supplied 

 by some opticians for this purpose a dark blue-green 

 between the microscope and the source of light. An effective 

 glass is to be obtained from Messrs. Hetley of Soho Square, 

 which entirely conforms to the required conditions, as it 

 appears simply to reduce the quantity of light transmitted 

 without in any way altering its colour-value. This is a matter 

 of considerable importance when colour-screens are in use ; for, 

 should a glass which has a power of selective absorption be 

 used, it would disturb the arrangements already made and 

 affect the resulting image. It is, of course, understood that 

 any light -modifier of this description is removed before the 

 actual exposure takes place. 



It is perhaps almost unnecessary to point out that the focus- 

 sing should as far as possible be carried out at the centre 

 of the field that is, with the centre of the field in the 



