490 SUMMARY OF CURRENT RESEARCHES RELATING TO 



suggested to the author by F. T. Cheshire, of the Birkbeck Institute. 

 It consists essentially of a glass disc of such diameter that it can be 

 dropped into the stop-carrier of the condenser with which it is used, 

 ruled with equidistant lines (a millimetre scale will be found as suit- 

 able as any). To use this piece of apparatus it is first necessary to 

 find the value of the scale when used with any given condenser. This 

 is effected by means of an objective of known aperture in the following 

 manner : — The condenser and objective having been focussed on an 

 object, the disc is inserted below the condenser, the eye-piece removed, 

 and the number of divisions of the scale visible in the field duly noted. 

 As in the case of the protractor apertometer, it will usually be necessary 

 to magnify the image so obtained in order to read the scale with accu- 

 racy. When set up in this manner the scale will appear sharply defined 

 right up to the edge of the field, provided that the aperture of the lens 

 measured does not exceed the aplanatic aperture of the condenser. Thus, 

 suppose the objective of known aperture to be a ^-in. of N.A. 0*34, 

 and the condenser to be the Abbe chromatic pattern N.A. 1*20 (this is 

 the total aperture, the aplanatic aperture is, of course, very much less, 

 approximately N.A. 0*50), then, proceeding as above, it will be found 

 that 8£ divisions are visible in the field, and that consequently 1 mm. 

 of the scale with this condenser has a value of N.A. 0'04. If now 



■ 



another objective, say a £-in., be taken and 5 divisions be found visible, 

 then the aperture will be N.A. ■ 20. 



The author gives a number of examples in which the apertometer is 

 used to obtain the conditions for good dark-ground illumination. 



Acetylene Gas for the Lantern. — T. D. Ersser states that he has 

 used acetylene gas for lantern purposes for the past two years. He 

 finds that the best apparatus is the Imperial cold generator which when 

 worked on the gasometer principle is perfectly safe. With 20 oz. of 

 the best calcium carbide and an argand burner, a light of over 300 

 candle-power, free from smoke and smell, and lasting for two hours, can 

 be obtained at a cost of ninepence. 



(4) Photomicrography. 



New Method of Focussing in Photomicrography.* — Katharine 

 Foot and Ella C. Strobell use a very simple form of vertical camera. 

 The Microscope (an ordinary Continental model with a direct-acting 

 screw fine adjustment) stands upon a base-board, 12 by 12 by £ in. 

 thick, to which wooden uprights, which hold a bellows camera, are 

 attached. 



In the new focussing method all need for a focussing rod, or other 

 appliance, to carry the movement of the fine adjustment screw to the 

 sensitive plate end of the camera is obviated, neither is it necessary to 

 project the image on to the ground-glass screen, for the correct focus is 

 obtained even before the camera is applied to the Microscope by the 

 simple expedient of using a spectacle lens of a certain negative strength 

 when focussing the instrument. 



The powers of the sjtectacle lenses suitable for given extensions 

 of camera, and other conditions, were experimentally determined and 



* Zeitschr. wiss. Mikr., xviii. (1902) pp. 421-6 (1 pi.). 



