ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 965 



of great interest, and has always attracted much attention. The 

 ingenious apparatus which he iises is shown in fig. 160 (drawn from 

 an imjjroved model made by Messrs. Watson and Sons. 



The base-plate is of ebonite, and has at one end a small pillar, 

 through which passes an arm carrying a piece of platinum. A similar 

 support at the other end carries a double disk, between the plates of 

 which are inserted the pieces of metal to be experimented on, viz. 

 magnesium, tin, brass, steel, carbon, lead, iron, copper, platinum, and 

 aluminium. The disk can be rotated by an ebonite arm so as to 

 bring each metal successively in line with the platinum. The 

 distance of the latter from the pieces of metal can be increased or 

 reduced by moving the lever-arm with which the platinum holder is 

 connected. A pint bichromate cell in connection with a small induction 

 coil is sufficient to actuate the apparatus. A 1 in., 1/2 in., or 2/3 in. 

 objective shows best. With a higher power than the 1/2 in. the 

 spark is likely to pass to the brass of the objective. 



The apparatus is adapted for use with the micro-spectroscope. 



Bertrand's Polarizing Prism.* — E. Bertrand proposes a new form 

 of polarizing prism as follows : — " The Nicol prism, the polarizing 

 apparatus most generally employed, is attended with certain dis- 

 advantages : (1) the obliquity of the end-faces in relation to the axis 

 of the prism; (2) the length of the prism, which is about four times 

 its breadth ; (3) the extent of the field, which is less than 30^ ; (4) 

 the necessity of employing a very clear and somewhat large piece of 

 Iceland spar, which is becoming more and more scarce and expensive. 



Hartnack and Prazmowski f have improved this apparatus ; the 

 end-faces of their form of prism being perpendicular to the axis, the 

 prism is shortened, and the field increased to 35°. 



In the Nicol prism, and in that of Hartnack and Prazmowski, a 

 luminous ray passing through the spar is divided in two : the ordinary 

 ray undergoes total reflection at the film of Canada balsam or linseed 

 oil, whilst the extraordinary ray is transmitted. By computation the 

 field, within the spar, cannot exceed 26° 33' 45" ; on emerging into 

 air the rays expand and the field attains 35°. It is impossible to 

 exceed this exterior angle by utilizing the extraordinary ray ; but if 

 the ordinary ray w^ere utilized, in consequence of its higher refractive 

 index, the field would be increased to 44° 46' 20" in air. 



To attain this result, I use a prism of flint glass of index 1*658 

 which I cut through a plane at an angle of 76° 43' 8" to the end-faces ; 

 the two section-faces thus produced are polished and between them is 

 placed a cleavage plate of spar suitably oriented, the whole cemented 

 together with a substance of refractive index equal to, or greater than 

 1-658. 



A ray of light entering the prism normally cannot traverse the 

 plate of spar without being divided into two rays, which are polarized 

 at right angles. The ordinary ray, whose index is 1*658, will 



♦ Ck)mpte8 Eendus, xcix. (1884) pp. 538-40. 

 t See this Journal, iii. (18^3) p. 428. 



