1906-1907.] Address by the President. 



Some sub-stage condensers have both these arrangements in 

 which case the cell in the turn-out arm is used for the 



tTeTXt 1 ^jf-\-!--^ ^- of glass foTmoX ^g 

 the light or of metal discs with small holes in different 

 positions for obtaining .W.-,.. illumination -that is a ray 

 of light passing obliquely through the object ^ 



The numerical aperture of the sub-stage condenser should 

 rrespond as nearly as possible to the\umerical aper u e 



Ln^olc^r%^Z ""'•■ ''' ^^°^^^^^^ should also b 

 <^^^roynaUc. On this point the late Sir David Brewster 

 said, ;'I have no hesitation in saying that the apparatul or 

 mummation requires to be as perfect as the apparatus for 



In using the sub-stage condenser, the first thing is to see 

 that It IS correctly centred with reference to the opti axis 

 the object-glass-that is, that the exact centre of'the len 

 of the condenser is in line with the centre of the obiect-llass 



ray^f Tht T' 'T ^^^^^^^^^ ^^ ^ smalltl^ e' 

 ray of light coming from the condenser. When the centring 

 13 made, open the aperture for the admission of as much "hf 

 as IS required, and then move the condenser up or down tS 

 the ^.^age of the la^np - fla^e or other source of ligul ac 

 this f^T"^ .^.. the oljeet under examinatfon. ItlsTn 

 this way the most perfect image is obtained. Do not u e 

 more light than is necessary. '^ 



In Fig. 1 Plate L, is represented the course of the rays 

 when the mirror only is used, l is the lamp flame, m the Z 

 side of the mirror, s the sub-stage condenser, and o the oWect 

 But when workmg with the sub-stage condenser, I prefer to 



bpfor r .11 ^"^^ P"'"^'^ "ojm^^n. Of a condensina.lens 

 before they fall upon the mirror. The manner of doin^ hi 

 was formerly explained. The course of the rays in such an 

 arrangement is shown in Fig. 2, Plate L., in which lTs the 

 lamp flame, c the condensing-lens, m the mirror s the sub 

 stage condenser, and the object. 



It was before strongly insisted on that, in order to render 

 he rays parallel, the lamp flame must be placed n the focu 

 and the centre of the condensing-lens. If these condition! 

 are not observed, a considerable l°oss of light is the resuuL 



