60 Scientific Intelligence. 



these bands as the temperature changes, the derivative of the 

 difference of the indices of the plate and the liquid with respect 

 to the temperature may be deduced. In the first series of experi- 

 ments a quartz plate, in the second a crown glass plate, was used. 

 The results of the four sets of measurements are given in a table, 

 the temperatures varying from 9° to 50°. With these are collated 

 the values previously observed by others. These results of obser- 

 vation agree well with those given by a curve of the third degree 

 calculated by the method of Cauchy, the equation of which is 



— = — 10~ 7 (125-46 -|- 41-285J — 0-01304* 2 — 0*00460£ 3 l. Inte- 

 dt v ; 



grating this expression, we have n = n — 1CT 7 (125'5£ + 20 # 642£ 2 



— 0-004352 3 — 0*001 lot 4 ) by which the index may be calculated 

 for any temperature. This equation gives 1*33397 as the value 

 of the refractive index at 0° and 1-32896 at 50°. Plates of fluorite 

 and beryl immersed, in water, at temperatm*es varying from 17° 

 to 37°, gave for the mean value of the derivative — 0-0000134, 

 whence n = n — 0'0000134£, in the case of fluorite; and in the 



dO 



case of beryl, — = 10' 7 (189-4 — 10-34£ + 0-2735^) for the ordi- 

 nary and — = 10~ 7 (180-3 — 10-3142 + 0'27B5f) for the extra- 



(Ait 



ordinary index. . The rate of variation of the indices of carbon- 

 disulphide, monobrom-naphthalene, turpentine and alcohol were 



determined by means of Talbot's bands. The first gave -y- 



dn at 



— —0-0006320 - 0-00000870^; the second — = - 0-00045748 



dt 



(group 1), - 0-00046404 (group 2); the third ~ = — 0-0005113; 



dn 



and the fourth -^ = — 0*0004179. In the case of the last three, 

 at 



it will be observed that the variation of the index is proportional 

 to the temperature. — J. Phys., II, iv, 389, Sept., 1885. G. f. b. 



6. On a Spectroscopic Optometer. — The difficulty of determin- 

 ing rigorously, by means of the existing optometers, the distance 

 of distinct vision in abnormal eyes, has led Zekger to the con- 

 struction of a spectroscopic optometer. Since with the ordinary 

 instruments, the recognition with any precision, of the exact posi- 

 tion of the tube at which the slit ceases to appear double, is well 

 nigh impossible, the author first sought to determine the accuracy 

 of the observation by using a lens made of Iceland spar, cut per- 

 pendicularly to the optic axis. With this lens the two positions 

 were observed in which the slit appeared single ; corresponding 

 to the ordinary and extraordinary foci respectively. From the 

 data thus obtained calculation gives two values of the distance of 

 distinct vision ; by the amount of disagreement of which, the 

 error of the method, can be ascertained. In this way it was shown 

 that errors of one or two centimeters even, may be committed 

 in measuring the eyes of aged persons. While using this appai-a- 



