18 OPTICAL PROJECTION 



abandoned now, because the aberrations are very imperfectly 

 corrected, and there is loss oi light by reflection at the edges, 

 the curve increasing the angle of incidence. The two 

 meniscus lenses of D were better, but never came into general 

 use, being superseded by F, a meniscus and double convex, 

 with the meniscus towards the radiant. This is known as 

 the Herschel condenser, being in general form modelled on a 

 burning-glass designed by Sir John Herschel, and was long 

 reputed to be free from spherical aberration, though it was 

 afterwards discovered that this supposition was due to an 

 error in calculation. As a burning-glass, however, such a 

 pair does give exceedingly good results ; and, correspondingly, 

 is excellently adapted for converting the light from a luminous 



Fro. 10 



point into a parallel beam. The light from a lantern has 

 however to be converged, except in optical experiments ; and 

 therefore this condenser is not so superior for lantern use ; 

 but it does well for lanterns used with the lime-light alone, 

 and is largely employed for such ; for larger radiants, like 

 lamps, it is not well adapted. Its best form is that devised 

 by Gravett, shown at a, where the meniscus next the light is 

 made rather smaller, and the second lens is of flatter curve 

 on the inner face. Optically, this is a very good condenser 

 for the lime-light, its principal defect being that the bulging 

 front prevents slides being brought up close to the margin, 

 and hence somewhat diminishes the size which can be illumi- 

 nated by a condenser of given diameter. 



