E. M. XELSON OX METHODS OF ILLUMINATION. 293 



ing of two doublets, forms the best possible condenser for low-power 

 work ; a blue-glass light-modifier for lamplight fits below these 

 lenses, and a ground glass (which is never used) fits above them 

 in place of the top lens. Even to-day no better condenser is 

 made for powers from in. downwards. Ground glass can be 

 used with or without a substage condenser, but we are told that 

 the orthodox method is to place a piece of finely ground glass 

 upon the stage immediately below the slip.* Ground glass scatters 

 the light it transmits in all directions, and therefore the objective 

 will be working at full aperture. So far it would seem that, 

 -with daylight and the concave mirror, all control over the work- 

 ing aperture is lost ; of course the light may be reduced by the 

 iris, but obviously there is no means of varying the W.A. to, 

 say, I or ^ cone. But if the unorthodox method of placing the 

 ground glass behind a substage condenser is adopted, we shall 

 find, by inspecting the back of the objective, that with the help 

 of the iris we can regulate the working aperture of the objective. 

 It would seem, therefore, that this position for the ground glass 

 is a better one than that usually recommended. 



With regard to its use with high powers the case is somewhat 

 different ; if with a -i-mm. apochromat a ground-glass screen, 

 placed immediately behind the object, be illuminated by a 

 substage condenser, and the iris fully opened, an image not very 

 dissimilar to that when ordinary critical illumination is employed 

 will be seen at the back of the objective (Fig. 4) ; but when the 

 iris is closed a marked difference takes place, for the image then 

 will not be of the ordinary form as in Fig. 5, but will be an 



under-corrected, but nevertheless it makes a useful condenser for ordinary 

 work with the medium powers (say, ^ and ^). When, however, the top 

 is removed, a perfectly achromatic combination is obtained, which is, as 

 I have already stated, the best ever constructed. Its modern substitute, 

 for which that particularly fine combination of Baker's may be taken as a 

 type, has also an uncorrected front lens ; but the backs are over-corrected, 

 and so the condenser as a whole is perfectly corrected. Now, when the 

 top of this modern condenser is removed you do not find such a perfect 

 low-power condenser as with the old form, because the combination is now 

 over-corrected. 



Invented by John Keates, of Liverpool. 



