418 Transactions of the Society. 



of a Nernst filament. If Nernst filaments could always be relied 

 upon, nothing could be better ; but, in fact, the Nernst filament 

 works under such severe limitations that it can only be regarded 

 as an article of luxury. The ordinary electric lamp is so very 

 much more trustworthy, and, for that reason, so much more con- 

 venient, that an illuminator, to be successful for general use, must 

 be designed for working with it. In the model now before the 

 Society the form of lamp adopted is the tubular form. This 

 enables one to place the ground glass end of the speculum within 

 half an inch of the glowing filament. If these tubular lamps 

 were made with still narrower tubes, so as to allow a yet closer 

 approximation of the ground-glass to the incandescent filament, 

 they would be even better for the purpose. In order to secure an 

 arrangement by which the light may be placed as near as possible 

 to the table, the lamp itself is arranged in a horizontal position, so 

 that the lamp-socket stands to one side, and nothing, save the wall 

 of the lamp-chamber, intervenes between the glass envelope and 

 the bed of the lamp-stand. 



With the same object of securing a low position on the table, 

 the lamp support is mounted on a pillar of special construction. 

 In theory this pillar is a girder made of three steel springs. In 

 fact, however, owing to the circumstance that I made it myself, 

 and under great pressure of time, its construction has been 

 simplified, so that only the flange members of the girder are in 

 use, the web member having been altogether omitted. The con- 

 sequence is that this pillar is slightly flexible in one direction, 

 and when the lamp is at its highest position it is liable to be 

 easily shaken. That, however, is not a matter of very great 

 consequence, because in its ordinary working position this mutilated 

 girder is strong enough. 



On the top of this pillar the lamp fitting is mounted in such a 

 manner as to have a power of swinging through an angle of about 

 fifteen degrees on either side of the horizontal position. A spring 

 actuated brake-block bearing on the side of the pillar serves to 

 give a holding grip which retains the fitting in any angular 

 position in which it may be placed. 



The lamp-chamber comprises an opaque screen surrounding 

 the lamp and attached to the side wall of the chamber, which 

 screen itself constitutes the top, bottom, back and front walls of 

 the chamber. The sixth side is left open, for the purpose of 

 ventilation, as the light which issues in that direction causes no 

 inconvenience to the user of the Microscope. The side-wall is 

 pierced by a circular opening into which a wooden plug fits. The 

 face of this plug internal to the chamber carries the lamp socket. 

 The reason for placing this socket on a circular plug, and not 

 directly on the wall of the chamber, is that in that way we obtain 

 the power of rotating the lamp about its own axis. An electric 



