ON CONSTRUCTING STANDS OF MICROSCOPES. 159 



may be instantly removed out of the way when not 

 wanted, without being detached from the bar. 



The large illuminating mirror should (if not of very 

 considerable dimensions) be made oval, and should work 

 upon a swivel joint at the bottom instead of the side, 

 otherwise it will be rendered unserviceable, for the long 

 axis of the ellipse cannot otherwise be brought into the 

 position necessary to give a round spectrum when placed 

 at the requisite angle for illumination, say 45 degrees. 

 The reason for the ellipticity of the mirror is this : An 

 image of it is constantly formed in the optic or visual 

 pencil, at the eye-piece, as may be seen with a magnifier 

 whenever bodies are viewed by the intercepted or trans- 

 mitted light which it furnishes. Now unless this image 

 is of equal size with the said visual pencil, an effect is 

 produced not very dissimilar to that of mutilating the 

 pencil in some other way, and exactly the same as to the 

 quantity of light lost. Now a round mirror (unless it is 

 of such size as to allow for the cutting off occasioned by 

 its angular position,) gives an elliptic pencil. But I 

 presume it is always advisable not to increase the bulk 

 of any part of an optical instrument when it can be 

 avoided ; consequently the true form for the mirror is an 

 ellipse. This form does not easily admit of the use of a 

 concave reflector; but I disapprove of them, as they 

 always seem to me to occasion a certain indistinctness in 

 vision by intercepted light, and if wanted only for illu- 

 minating opaque objects, by the help of silver cups, may 

 be dispensed with ; for a much better action is produced 



