160 MICROSCOPIC ILLUSTRATIONS. 



by a plane mirror, co-operating with a condensing lens. 

 Because an angular plane mirror and a condensing glass 

 produce a round spectrum ; whereas a concave mirror, 

 when tilted, always gives an oblong one ; however, in 

 the present construction, I always suppose that when 

 silver cups are employed, the condenser is to act by itself 

 without any mirror at all, by which arrangement the 

 maximum of illumination is given. 



The size of the mirror is determined by its distance 

 from the object-glass, or single magnifier used, and by 

 the angle of aperture possessed by them. The longer 

 the bar, and the greater the aperture of the optical part, 

 the larger must be its dimensions to fill up the visual 

 pencil. 



The reverse of the mirror will be best occupied by a 

 surface of plaster of Paris, to reflect the light of the sun, 

 which will be found of great use in ascertaining the true 

 tints of transparent objects, the colours of which are 

 shewn by this sort of light with the utmost truth and 

 delicacy, far better than when brought out by a piece of 

 ground glass interposed between them and the polished 

 mirror. The double action of the mirror will be com- 

 pletely commanded by the most inexperienced observer, 

 if a milled head is attached to its transverse axis, as will 

 be shewn in its proper place. 



Now these are, I think, the main and capital points to 

 be attended to in the construction of a stand. As, how- 

 ever, every question is said to have two handles to it, I 

 shall endeavour to discuss the merits of some opposite 



