102 PRACTICAL MICROSCOPY. 



extreme importance when we consider the heat given out 

 during combustion in most oil lamps. 



These remarks upon the behaviour of certain lenses and 

 mirrors towards the rays of light may be considered super- 

 fluous, nevertheless, as the student proceeds, he will find 

 that not one word too much has been written. False 

 appearances are often produced by the bad employment of 

 light, and the student is advised to practise many kinds of 

 illumination upon objects with which he may be familiar, so 

 as to acquaint himself with the various appearances which 

 diverse applications of it will afford. 



When the action of the various mirrors and lenses has 

 been fairly grasped, the student should proceed with some 

 work capable of giving him experience of the manner in 

 which various objects are delineated or depicted under 

 various objectives. In the days of Dr. Goring (1832) when 

 objectives even of high amplification had not surpassed an 

 air angle of 55, and even when achromatics were despised 

 by nearly all working microscopists, it would not have been 

 a difficult task to test an objective for its spherical and 

 chromatic aberrations, by means of the tests we now possess ; 

 but time has changed all things microscopical ; really bad 

 lenses are rarities, and taking objectives to, and including 

 the -inch, it is remarkable how few imperfections they 

 possess. 



In order to correct perfectly the aberrations of objectives, 

 the practical optician employs the globule of mercury or 

 "artificial star" as a test object, while the accuracy of their 

 setting is examined by studying the reflected image of a 

 flame or the window bars, while the mount with lenses 

 in situ is revolving in the lathe. 



Beyond mention of it, the " artificial star " test need not 

 be described here; it has been fully treated upon by 

 Dr. Goring in the 'Microscopic Cabinet/ to which the 



