420 Transactions of the Society. 



lamp does not, in fact, shed light equally in all directions. The 

 convolutions of the filament by reason of their more or less shading 

 one another, cause the light to be unequally distributed in various 

 azimuths, and yield, moreover, unequal reflections in various direc- 

 tions. And then, again, the filament itself is never quite accurately 

 centred in the lamp, and for these reasons it is desirable to be able 

 to adjust the lamp by rotation about its own axis into that position 

 in which it sheds the maximum amount of light upon the 

 speculum. 



The front wall of the lamp-chamber is pierced by a circular 

 aperture to which a short tube is fitted. This tube is, for the sake 

 of lightness, made of cardboard. It serves to prevent any light 

 reflected from the interior of the lamp-chamber from reaching the 

 eye of the observer at the Microscope. Through this tube and the 

 aperture to which it is fitted the speculum extends backward in the 

 direction of the lamp. At its fore end the speculum is mounted in ' 

 a suitable fitting, so as to extend horizontally in front of the lamp. 

 In order that the intensity of the light may be varied, the chamber 

 containing the lamp is itself mounted upon a slide provided with 

 a rack in which a pinion works. This slide is an extension of the 

 side wall of the lamp-chamber, and is the sole support of the lamp- 

 chamber and lamp. A milled head enables the operator to act 

 through the pinion upon the rack, and by varying the distance- 

 between the ground-glass end of the speculum which is fixed and 

 the lamp, which is thus made movable, to vary the strength of the 

 light without varying its angle of incidence. 



The speculum is mounted, as already stated, so as to be fixed 

 in relation to the fitting. Substantially its mount consists of a 

 tubular fitting in which the speculum itself is held, the tubular 

 fitting being inserted into a flanged opening in a plate standing at 

 right angles to the axis of collimation. This plate forms a rigid 

 member of the central part of the fitting which is mounted, as 

 already stated, so as to swing about the pillar for the purpose of 

 collimating the speculum. An adjunct which can be fitted over 

 the front of the speculum carries the bullseye and a cell for 

 mounting stops and diaphragms when it is desired to modify the 

 form of the light source. The bed of the instrument is so arranged 

 that the milled head by which the lamp is raised or lowered stands 

 well to one side, so as to be conveniently accessible to the operator's 

 right hand. This little arrangement is, in fact, of capital import- 

 ance. A lamp fitting which requires the operator to put his hand 

 close to the lamp itself is for that reason alone impracticable. If 

 his hand has to come into proximity with the lamp he must use his 

 eye to guide it. In this instrument the handle, by which the lamp 

 is raised or lowered, stands well clear, in a place by itself, where 

 the observer can reach and operate it without fear of striking or 

 disturbing anything else. 



