C. S. Hastings — Double Refraction in Iceland Spar. 65 



between R and the narrow cleavage face at b ; and finally, 

 the normal angles between the cleavage faces a b Q and be Q 

 respectively. The precision of all the measures involving re- 

 flection from a surface of cleavage is of course much inferior 

 to those made upon the polished: surfaces. The first group 

 gives the refracting angles, and the others only serve to deter- 

 mine the direction of the crystalline axis, a datum not used in 

 the final reduction but useful as a check on the' work. 



The general method of determining these angles was as fol- 

 lows : The telescope replaced the fixed collimator which was 

 removed. By means of a plate of plane parallel glass and a 

 quasi collimating eyepiece* the axis of the telescope was ren- 

 dered strictly perpendicular to the axis of rotation of the 

 instrument. The focal adjustment of the telescope could be 

 made at the same time with great precision : magnifying power 

 used, twenty diameters. Following this adjustment the glass 

 plate was replaced by the prism, which was so adjusted that 

 the line of collimation fell close to the center of each face when 

 in position for observation.- That this condition, a most im- 

 portant one, was fulfilled, was determined by removing the 

 ocular and looking at the prism through the telescope tube. 









Table 



I. — Anglei 



of 



prism - 



= a = 60. 







PQ 



PR 



QR 



Obs. 



t 



Red. 



Obs. 

 — 2 / -516 



t 



Red. 



Obs. 



t 



Red. 



+ 1/-205 



17 



2 



+ 1/-285 



17 







— 2'-519 



+ 1'330 



16-5 



+ l'-303 



•242 



17 



1 



•280 



•492 



17 







•519 



•298 



16-5 



•303 



•273 



17 







•276 



•521 



17 







•519 



•254 



16-6 



•293 



•407 



19 



8 



•421 



•521 



19 



55 



•394 



•067 



19-2 



•046 



•421 



20 







•412 



[•652] 



19 



65 







1-141 



19-25 



•041 



•432 



20 







•412 



•418 



19 



75 



•384 



0-908 



19-55 



1-013 



•391 



20 







•412 



•350 



20 



7 



•338 



•895 



210 



0-875 



•469 



20 



1 



•417 



•315 



21 







•323 



•883 



210 



•875 



•453 



20 







•412 



•225 



21 



5 



•299 



•858 



21-5 



•828 



•390 



20 







•412 



•235 



21 



3 



•308 



•836 



2.1-1 



•866 



475 



20 



1 



•417 



•350 



21 



1 



•318 



•842 

 •873 



21-1 



•866 



•514 



20 



1 



•417 



•354 



21 



1 



•318 



21-2 



•856 



•470 



21 







•457 



•306 



21 



2 



•313 



741 



22-5 



•732 



•484 



21 



2 



•466 



•231 



20 



8 



•333 



■703 



22-6 



•723 



•481 



21 



1 



•462 



•242~ 



22 



9 



•230 



+ 0-695 



22-8 



+ 0-704 



•453 



21 



7 



489 



•247 



23 







•225 





• 





•427 



21 



7 



•489 



— 2 -247 



23-0 



— 2 -225 









•478 



21 



6 



•485 















•519 



21 



6 



•485 















•445 



21 



6 



•485 















•435 



21 



8 



•494 















•511 



23 







•548 















•511 



23 







•548 















+ 1 -528 



23-0 



+ 1 -548 















* This is described in the paper " On the influence of temperature on the opti- 

 cal constants of glass." This Jour., Ill, vol. xv, p. 271. 

 Aii. Jour. Sci.— Third Series, Vol. XXXV, No. 205.— Jan., 1888. 



