1902.] Refractive Indices of Fluor ite, Quartz, and Calcite. 330 



271 28 39-0 208 



/3 (240°) 28 13-75 



27 55 -25 



C. 31 29 37*375 



Result. 



Dev. = 31° 29' 36" -208. Ref. Index = 1 '4325233. 

 The above is an exact copy of the original observation for line C (hydrogen a). 

 It will be seen that the readings of the micrometer are arranged in three groups, 

 each of which is complete in itself, since it contains readings at all the angles and 

 at each of the three positions of the circle. The averages for each group, called 

 A, B, and C, are underlined twice, and the final result is the mean of these three 

 averages. The three angles of the prism are called a, /3 and y : each of these gives 

 the deviation under measurement three times, that is, once at each of the three 

 positions of the circle (zero = 0°, 120° and 240°). The two burrs thrown up by the 

 engraving tool on each side of every division, already described, give the first 

 readings, and in taking the mean of these a deduction of 1" for every 1', measured 

 in the microscope, is made for the optical error of the reading microscope, a careful 

 measurement of the error having come out within a small fraction of this. The 

 microscope was furnished with a fixed and a movable quartz fibre ; sometimes the 

 movable fibre was too close to the other to read with accuracy, and then had to 

 be moved off to the next division of the circle (5' further off centre). In this case 

 I have marked the reading with a x , so as to ensure attention being called to the 

 matter. After turning the same angle of the prism round to the other side as 

 usual, another mean is obtained from two burr readings in the same way. These 

 two means, one on each side, underlined once, give by the usual method the value 

 for each angle in the groups already described. 



Appendix II. 



Table giving the Focal Lengths in metres of a Compound Lens of 

 Fluorite and Quartz achromatised for Wave-lengths 7950 and 1852. 



Radii. — E = 0-28358015, S = - 0-20801615, E' = S, S' = oo , 

 E, S, E', S' refer to the surfaces of the two lenses. 



Wave-length. 



Focal length. 



"Wave-length. 



Focal length. 



7950 



1 -ooooo 



2749 



-99382 



7652 A' 



1 -00005 



2573 



'99314 



7065 B' 



1 -00063 



2445 



-99275 



6563 C 



1 -00073 



2313 



-99208 



5893 D 



1 '00081 



2265 



-99225 



5607 A 



1 -00076 



2194 



0-99233 



5270 E 



1-00061 



2144 



-99267 



4861 F 



1 -00022 



2099 



-99281 



4341 G' 



-99945 



2062 



-99299 



3962 



-99857 



2024 



-99356 



3611 



-99750 



1988 



-99461 



3303 



0-99654 



1933 



0-99563 



3034 



-99548 



1852 



1 -ooooo 



Graphical representations are given in the curves on p. 333, the 

 achromatised focal length being there 6985 instead of 1000 mm, 



32 6-5 



31 43 25 



31 23 -5 



