1082 RECORD OF CURRENT RESEARCHES, ETC. 



aperture of tlie diapliragm being adapted to the maximum thickness 

 of beam which can be effective for illumination, and which (calling a 

 the aperture of the lens and i the angle of incidenc3 of the beam) 

 = a cos. i ; or for an aperture of f inch ( = 0*875 inch) and an inci- 

 dent angle of 62°, 0"4:11 inch, or roughly 0*4 inch. 



The less the incident angle, of course the larger the beam of light 

 will be, and the greater the diameter of the diaphragm. The refractive 

 index of the glass employed to make the plate beiug 1 • 5, in order 

 that the critical angle 41° 48' may not be exceeded in the refracted 

 ray, this angle of incidence or i must not be less than 61° 51', or 

 roughly 62°. 



This minimum value of i determines the area of surface which 

 can be illuminated on the Microscope stage, but by altering the angle 

 of the mirror very slightly, all parts of the object may be successively 

 projected on the screen. This minimum value is easily obtained 

 from the critical angle of the glass employed, which is 41° 48'. The 

 complement of this, or 48° 12', is equal to the angle of refraction 

 (or r) when the minimum value of i is attained. 



'4^^'- 1-5, 

 sin. /• 



sin. 8 = 1-5 (sin. 48° 12'), 



i = er 51'. 



In other words, the angle between the luminous ray and the glass 

 plate can never exceed 28° 09', or in round numbers 28°." 



Ebonite in Microscopical Appliances. — In America ebonite has 

 been adopted for some years for mounting eye-pieces, and for stages 

 of laboratory Microscopes, principally by the Bausch and Lomb 

 Optical Company, who claim for it sjijecial adaptability for these pur- 

 poses as well as economy. M. Verick, of Paris, has also used it for 

 the diagonal sliding-boxes containing the prisms of his binocular eye- 

 piece and the outer plates into which they fit, and it has also been 

 adopted for rings for cells. 



We are glad to see that ebonite is coming into use in this country, 

 having been adopted for Stephenson's safety-stage* (by Mr. Teesdale), 

 and now for Botterill's life trough, f There are many other pieces of 

 apparatus for which the use of ebonite would be a great advantage in 

 reducing weight. 



* See this Journal, ante, p. 332. f Ibid., p. 148. 



