ON THE ILLUMINATION OF OBJECTS. 189 



plate of glass, like that of the ordinary mirror, but, instead of 

 the silvering, he coats the back with white zinc paint. This 

 mirror is set in a frame, and when required can be placed 

 over the silvered one; its eflfect on certain objects, such as 

 sections of bone and teeth, is very striking. 



Sir David Brewster, to whom we are indebted for so many 

 valuable suggestions and discoveries in optical science, states* 

 that, for perfect definition, the focus of the illuminating lens 

 for transparent objects should fall exactly upon the object, so 

 that there shall not be two sets of rays at different angles, 

 one proceeding from the luminous body, and the other from 

 the object to be magnified ; and adds, — " That the illuminating 

 lens should be perfectly free from chromatic and spherical 

 aberration, and that the greatest care should be taken to ex- 

 clude aU extraneous light, both from the object and from the 

 eye of the observer." All this implies that the condenser 

 employed should be in itself achromatic, for an ordinary lens 

 acting more or less as a prism to decompose the light, it 

 follows that in most cases we shall have the object illuminated 

 with the prismatic colours, which would injure the sharpness 

 of outline. Hence the necessity of employing a doublet or 

 some kind of achromatic lens for a condenser. 



Achromatic Condenser or Eclairage. — This instrument, the 

 invention of M. Dujardin, an eminent French microscopist, 

 has been previously alluded to ; the different forms of it, sup- 

 plied with our best microscopes, are represented by figs. 58, 

 59, 60, and although differing slightly in certain points of their 

 construction, they all, nevertheless, are capable of being so 

 adjusted for use, that the axes of the illuminating lens and 

 object-glass may accurately coincide with each other. The 

 means provided for effecting these adjustments are represented 

 by the- figures; but, in order to facilitate their use, Mr. Ross 

 has drawn up a series of directions, which, being applicable to 

 other microscopes as well as his own, I here quote in full : — 



"When employing this apparatus, the general practice is 

 to insert in it, as an illuminating lens, the object-glass next 



* Treatise on Microscope, p. 146. 



