The Helmholtz Theory of the Microscope. By J. W. Gordon. 445 



Experiments shown at the Meeting. 



'- « 1. Showing the dependence of the diffraction pattern upon the radius 

 of curvature of the wave-front which passes the aperture. — In this 

 experiment a diffraction grating is mounted on the stage. The object 

 is the image formed by the condenser of the slide mounted in front of 

 the lamp. By moving the condenser up and down its focal plane can 

 be made to coincide with the diffraction grating on the stage or to lie 

 beneath it ; the distance between the focal plane and the diffraction 

 grating being, of course, the radius of curvature of the wave-front. It 

 will be seen that as the focal plane approaches the grating the displace- 

 ment due to diffraction diminishes, and when the two coincide it 

 disappears. The two diffraction gratings which lie side by side on the 

 stage are ruled, one with spaces double the breadth of the spaces of the 

 other. See above, p. 410. 



2. Invading antipoints. — In this experiment a shutter is arranged 

 behind the objective, which, by turning the handle on the base-board, 

 ■can be gradually introduced into the tube of the instrument. It is to be 

 noted that the advancing shutter destroys the resolution of the image 

 by cutting down the eccentric beams, so that its effect is produced upon 

 the centre of the field before the edge of the shutter itself appears in 

 the instrument. See above, pp. 411 to 413. 



3. Oscillating screen ; electrically actuated. Designed and exhibited 

 by Mr. E. Russell Clarke. — In this experiment the oscillating screen is 

 driven electrically at a very high speed ranging, according to the number 

 of oscillations in the electrical supply, at somewhere about 80 excursions 

 a second. Note in this case the entire absence of any visible trace of 

 the screen due to the high speed of vibration. Note further that the 

 high speed makes for the steadiness of the instrument as the oscillation 

 is too rapid to be sensibly communicated to the tube. In order to 

 secure this freedom from vibration the mounting in which the screen 

 oscillates is made heavy so as to take up its i vibrations, and is supported by 

 means of rubber washers upon the Microscope frame. See above, p. 420. 



3a. Oscillating lamp filament. — The Microscope in Experiment 

 No. 3 is illuminated by an electric lamp whose filament is fixed on the 

 screen. The filament is caused to vibrate at a high periodicity, thereby 

 producing a brilliant surface of focal light, instead of a straight line. 



4. Oscillating screen driven by a spring motor for visual demon- 

 stration. — In this experiment the speed of the oscillating screen is com- 

 paratively low — about three excursions a second. The object exhibited 

 is P. any. magnified about 8000 diameters. Side by side with this 

 Microscope there is arranged a twin instrument having exactly the same 

 optical arrangements, except that it is not provided with a screen, giving 

 the same magnifying power therefore, and exhibiting the same diatom. 

 Note the imperfect definition of the aerial image and the number of 

 intrusive spots, and compare with it the clean appearance and sharp 

 definition of the screen image. 



5. Oscillating screen for photographic use. — In this experiment the 

 compounding draw-tube takes the place of the optical bench, and a 

 magnification of about 400 diameters is obtained direct, and the negative 

 so formed will usually bear enlargement four or five times in printing. 



Any. 19th, 1903 2 G 



