The Helmholtz Theory of the Microscope. By J. W. Gordon. 421 



diameter, say to the y^^th part of an inch, a very small obstacle 

 will wholly obliterate the light from a point on the object. Now, 

 the obvious remedy for this defect is to spread out the contracted 

 wave-front. And this is quite feasible. A reflecting or refracting 

 screen interposed in the focal plane will scatter the light over as 

 wide an angle as you please, and clear the image entirely of these 

 intrusive shadows. You will this evening have an opportunity of 

 judging for yourselves how great an improvement may thus be 

 effected in the appearance of a highly magnified image, for in one 

 of the Microscopes upon the table you will find a Pleurosigma 

 angulatum magnified about 8000 diameters and thrown upon a 

 ground-glass screen. Another Microscope standing beside it and 

 fitted up as a twin instrument exhibits for comparison another 

 specimen of the same diatom under the same magnifying power, 

 but without the screen. It will of course occur to you that the 

 grain of the screen must be exhibited as well as the detail of the 

 picture, since both are focussed in the same plane, and this would 

 inevitably be the case if the screen were at rest. To obviate this 

 inconvenience it is kept in rapid, oscillation in a more or less 

 elliptical orbit. The movement, which is actually about three* 

 oscillations a second, is too rapid to be followed by the eye, which 

 thus receives the impression of a delicately shaded picture thrown 

 upon a structureless screen. In this way the picture can be 

 magnified to any extent without impairment from the shadows of 

 extraneous bodies. 



Such an oscillating screen is capable of being made equally 

 useful in photomicrography if the appliances are employed to 

 which I propose presently to refer. But for that purpose the 

 screen may oscillate much more slowly since the sensitive plate 

 accumulates impressions during a comparatively long exposure. 

 There is on the table a Microscope fitted with what I may perhaps 

 -call a compounding draw-tube for producing photographs and 

 there you will see that the screen makes only about ten excursions 

 in a minute. There is an advantage in this slow oscillation as it 

 minimises the danger of shaking the instrument by the motion 

 of the screen. 



The Compounding Draw-tube. 



(2) This leads me to speak in the second place of the appliances 

 at present in use for photomicrography. Everybody must have 

 been struck by the contrast, which the compact form of a modern 

 high power Microscope presents to the clumsy appliances, considered 



* Subsequent experiment has shown that higher frequencies of, say, six to ten 

 ■oscillatious a second, give visibly better results than three oscillations if the illumi- 

 nation be brilliant. 



