The Eye and the Microscope. 429 



to use those mentioned well, and they must, under any circum- 

 stances, form the staple instruments of research. The lowest of 

 these powers, the inch, or two -thirds, should have a perfectly 

 flat field, and evince no sensible defects if made to give four 

 times its ordinary magnification by a higher eye-piece and a few 

 inches of the draw-tube. A well-made section of an echinus' 

 spine is a good test for flatness of field — the margin and the 

 centre should come in and go out of focus together. That 

 elegant Diatom, the Arachnoidiscus Ehrenbergii, will likewise 

 supply a fair but by no means a difficult test of defining power. 

 Let us imagine a beginner trying to show these two objects. 

 In the first place, let him take the echinus' spine, the inch or 

 two-thirds object-glass, and the first eye-piece. Having got 

 the spine in focus, the mode of illumination will decide whether 

 it can be seen without wearying the eye. The microscope ought 

 to have the mirror below the stage so mounted as to move up 

 and down, and right and left, and to assume a position at any 

 angle with the plane in which the object lies. Candles should 

 be avoided as a source of light. They are too unsteady and 

 continually vary in height. Discard them, therefore, and 

 buy a small paraffin lamp for a shilling or eighteenpence.* It 

 will be high enough for many purposes, and can be raised when 

 required by a block of mahogany or a book. As a general 

 rule, the source of light is required at two elevations only — one 

 suited for transparent illumination with the rniiror under the 

 stage, and the other to enable the bull's-eye to condense the 

 rays upon any substance to be seen by the light which it 

 reflects. The lamp should, except in some special case, be 

 placed on the left of the observer, a few inches from the micro- 

 scope ; and to prevent his eye being distracted by its glare, a 

 shade should be employed. This is made of various patterns 

 to suit different fancies, but the simplest plan is to take a piece 

 of thin flexible cardboard, about nine inches by six, cover it 

 with black cotton velvet, which has no lustre, and make a hole 

 in it through which the tube of the microscope, immediately 

 below the eye-piece, can be introduced. Supplied with this 

 screen, both eyes should be kept open, and the object steadily 

 viewed as soon as the focus has been accurately arranged. 

 Then take the mirror, which we will suppose to have been 

 turned so as to throw some light through the echinus' spine, 

 and observe the effect of slight changes in its position. The 

 whole field should be equally lit, or fatigue will ensue from 

 some portions being seen worse than others. There should 



* Many observers will prefer a superior lamp, and they should inspect the 

 elegant and excellent pattern devised by Mr. Pillischer, the optician. In addi- 

 tion to being good for the microscope, it is one of the best reading-lamps yet 

 introduced. 



