768 SUMMARY OF CURRENT RESEARCHES RELATING TO 



If the curvatures corresponding to these rays be A, B, C, then- 



A = {(> 3 - /a 2 ) 0*' 2 - /*'i) " O^s-^'s) (H - /*iM 0*a" _1 ) 

 = 0-0000002549117 



B = { 0" 3 ~ /*" a ) (/^2 ' ■ /*i) " (/*3 - A*a) (^"2 ~ / x "1) J 0*'a ~ X ) 

 = 0-0000007308256 



C = I (/*' 8 - j*' 2 ) 0" 2 - /*"0 - 0*"a " /A) 0*'a " /*'i) J 0*a '" *-) 



= 0-0000005904547 



fx' 2 - 1 



Hartmann, J. — Die Korrektur des potsdamer 80-cm. Objectivs. 



[The author describes how the constructional errors of this large lens were 

 gradually reduced.] 



Zeitschr. f. Instnimentenk., xxix. (1909) pp. 217-32 (1 pi. and 6 figs.). 



(.3) Illuminating- and other Apparatus. 



Arrangement for Microscopical Observations at Extreme Tempera- 

 tures.* — H. E. Boeke describes the following apparatus intended for 

 use with the ordinary polarising Microscope. It admits of observations 

 and measurements in parallel and convergent polarised light, strong 

 carbonic acid and ether, or liquid air being used as the cooling medium. 

 The apparatus (fig. 120) consists of two cylindrical glass troughs G 19 G 2 

 of about 8 * 5 cm. diameter, set one above the other. Two thin-walled 

 central glass tubes form the observation tube, and are made in one 

 piece with the troughs. The height of the lower trough is 5 cm., and 

 that of the upper 4 cm. These troughs serve for the reception of 

 carbonic acid and ether or of liquid air. Quartz-glass for the troughs 

 is to be preferred on account of its durability, in spite of its high cost. 

 Both troughs are inclosed in a highly .polished nickel casing, in two 

 parts, so as to reflect off any external heating influences. The upper 

 part of the nickel casing N x slips over the lower N 2 , and contains a 

 nickel floor b b with an asbestos layer, on which the upper cooling trough 

 rests. A slide s of nickel plate is placed in this floor, and receives the pre- 

 paration. A slit made in the upper nickel casing is large enough to admit 



* Zeitschr. f. Instnimentenk., xxix. (1909) pp. 72-4 (1 fig.). 



