THE MICROSCOPE AND ITS PARTS. 
27 
part of a stand as sold, and in certain circumstances, 
as when we are working with the stage horizontal, the 
mirror is of importance. This matter will be discussed 
later. 
The beginner must examine the optic tube of his in¬ 
strument, and make sure that the interior has no shiny 
spots or areas ; the inside of the tube should either be 
thoroughly coated with a dead black, or lined with 
black velvet. Any shiny parts will lead to trouble in 
some of our after performances. 
En resume: A stand for photo-micrography must be 
absolutely rigid and steady ; its fittings and “ motions ” 
must be of the best workmanship; it must be capable 
of being used in the horizontal position; it must have 
a good substage, focussing, and centring; it may with 
advantage have a mechanical stage, a good one, if any; 
and its tube must be dead black inside. 
The length of the tube is a matter on which opinions 
differ; Continental workers generally use a tube of 
about 6J inches, the English size being about 10 inches 
—160 and 250 millimetres respectively. Some makers 
think to accommodate all clients by making a short 
tube and putting to it a considerable length of “ draw”; 
but this, sooner or later, leads to trouble, for the draw- 
tubes work loose in time. But one length of draw is not 
only permissible, but, under certain circumstances, neces¬ 
sary, as where we use the higher power apochromatic 
objectives. Consequently we recommend one draw, 
and this is most conveniently worked by a rack and 
pinion, as seen in our fig. 1. Objectives are usually 
