New Illuminator for the Microscope. By J. W. Gordon. 421 



To summarise the advantages which this form of lamp pre- 

 sents : — 



1. It has the capital advantage of the speculum, that is to say, 

 the light-source is a circular, structureless disk, which can be made 

 to give either as large or as small an illuminated field as is desired, 

 and which, whatever the focusing of the condenser, never pro- 

 duces unsymmetrical diffraction and never exhibits any disturbing 

 structure. 



2. The intensity of the light is completely under the operator's 

 control, and he can either increase or reduce it within very wide 

 limits without producing any alteration in the angle under which 

 the light falls upon his object. In practical working this is a 

 matter of very considerable importance, because the observer can 

 adjust the brightness to suit his eye without the penalty of altering 

 the focal adjustment of his system. He is thus not bound to work 

 with excessive light when working with critical illumination, and 

 need not incur the fatigue resulting from that cause. 



3. The adjustability of the lamp as to height enables the 

 centring to be done with the utmost precision. It is astonishing 

 with what precision a mirror can be angularly adjusted by hand 

 for the purposes of positioning the source of light. It is less 

 astonishing but much more annoying to observe how difficult it is 

 to get a mirror to keep the position assigned to it. It is hardly 

 possible, having gripped a mirror for the purpose of adjustment, 

 to let it go without disturbing the adjustment made. For this 

 reason it is immeasurably easier to make the fine-adjustment by 

 means of a rising and falling lamp than by manipulating the 

 mirror and, in fact, for really accurate centring, an appliance such 

 as this lamp possesses is indispensable. 



4. A merely mechanical convenience, but one which every 

 mechanician will appreciate, is the absence of guides for con- 

 trolling the movements of the lamp-chamber. The pillar, which 

 is stiff enough to make guides of overhead construction super- 

 fluous, but which at the same time can rise and fall so effectually 

 as to bring its burden right down into contact with the bed of the 

 instrument, and can do this without going through the table on 

 which it stands, is a mechanical novelty which will probably find 

 useful application in other connections beside that to which it is 

 ■here applied. 



