814 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



This is apparently the only inconvenience associated with the burner, and 

 is more than compensated by the advantages of the light. 



"Old and New Microscopical Instruments."— Apparatus for testing 

 Refractive Index.* — The text of Dr. G. Martinotti's article under the above 

 quoted title is that there is nothing new under the sun, and as here applied, 

 he remarks how frequently the newer apparatus is but an improvement on, 

 or a perfectioning of, some older instrument. As au example, he refers to 

 Prof. H. L. Smith's apparatus for determining the refractive index of 

 liquids.| 



Between two plates of crown glass is formed a space, one side of which 

 is flat, the other concave. When the cavity is filled with a liquid with 

 higher refractive index than that of glass, a plano-convex lens is the result. 

 By means of a simple device this artificial lens is fitted behind the weak 

 objective of a compound Microscope, so that the two form an optical system. 

 Then, according to the difference in the refractive index of the interposed 

 lens, the image of the object examined falls at a different distance, and the 

 amount of displacement imparted to the optical arrangement in order to see 

 the object clearly gives the refractive index of the liquid under examination. 



The author then remarks that the principle had been previously applied 

 for the same purpose, but in a somewhat different manner. The reference 

 is to Sir D. Brewster's ' Treatise on new Philosophical Instruments for 

 various purposes in the arts and sciences,' Edinburgh, 1813. In this, at 

 p. 240, will be found the ' Description of an instrument for measuring the 

 refractive power of fluids. . . .' There Brewster refers to the fact that 

 Euler had already conceived the notion of determining the refractive indices 

 of liquids by inclosing them between two lenses (menisci). This idea was 

 carried out, though imperfectly, by his son. Brewster's device was as fol- 

 lows fp. 247) : — In the extremity M N of a Microscope fitted with its 

 objective is placed a thin plate of glass a. The biconvex lens h is fixed to 

 the end of a short tube A B C D, screwed on to M N so that the internal 



Fig. 225. 



surface of the lens could be made to touch the plate a, or removed away 

 from it. In A B C D, just behind the lens h, are two holes for the intro- 

 duction of fluids into the cavity between a and h. Thus is formed a plano- 

 convex lens which can be diminished or increased in size by altering the 

 position of the screw. 



The plano-convex lens increases the focal length of h, and therefore 

 forms the image of any object m, at a greater distance from the point P, 

 situate at the anterior focus of the ocular Q R. But as the lenses Q R and 



* Zutschr. f. Wiss. Mikr., iii. (188G) pp. 320-30 (1 fig.), 

 t See this Journal, 1885, p. lOtiG. -. 



