The Hclmholtz Tlieory of the Microscope. By J. W. Gordon. 4.25 



invading antipoints to disturb the image formed in the middle of 

 the field. It is well known that under these conditions low- 

 angled objectives will give surprisingly fine resolution, the ex- 

 planation being that all the antipoints, although large, are very 

 approximately uniform in shape and size, and consequently no 

 false lights obscure the shadows forming the image. 



Limitation of the Illuminated Area. 



By analogy to this known contrivance it seems to follow that 

 resolution in the central region of the field may be promoted by 

 any artifice which diminishes the brightness of the peripheral 

 parts. In that case the shape of the source of light will appear 

 to be a matter of importance, and a small disc of light which can 

 be focussed upon the exact part of the object which we desire to 

 examine, leaving the rest of the field in darkness, will be the best. 

 From this point of view the edge of a lamp flame will be better 

 than its broad side, but a spot of light— as on a lime cylinder,, 

 better still. Experiments fully substantiate this inference, and on 

 the table to-night you will find a lamp so arranged as to illustrate 

 the point. Two shutters are disposed in such a way that the one 

 or the other can at will be interposed between the flame and the 

 Microscope. The one shutter exposes so large an illuminating 

 surface as to flood the whole field of the instrument with light ; 

 the other is pierced by a minute aperture which shows as a 

 mere spot of light in the field, and must be moved to the part of 

 the object which it is desired to examine. You will observe that 

 the introduction of marginal light sensibly impairs the resolution 

 at the centre of the field. Now this contrivance can be employed 

 with wide-angled objectives, whereas the method of dark field 

 illumination breaks down with them. This then is a peculiarly 

 useful expedient since it enables the microscopist to use to the 

 best advantage his objectives of highest power. Moreover, it 

 would seem that, excellent as is the flame of the Microscope 

 lamp it is not ideally perfect as a source of light, and that some 

 convenient form of light-source which will emit a sufficient, 

 amount of light from a smaller and more symmetrical radiating, 

 surface is still to be desired. 



The Rating of Objectives. 



Finally, there is the very important question of the rating of" 

 objectives. A sketch has been given above of the method proposed 

 — or to speak perhaps more accurately, suggested — by Helmholtz. 

 The suggestion is scarcely in a shape sufficiently elaborated to be- 



