492 



Sir J. Conroy. 



[Mar. 3, 



numbers that agree very fairly well with those determined in the 

 above-mentioned manner, especially "when the actual weight of the 

 gold (in no case exceeding -0029 grm.) and the uncertainty as to its 

 true specific gravity in the state of gold leaf is taken into account. 



The glass slips were fastened to the goniometer, and the constants 

 determined for the gold-air surface ; four observations being made in 

 each position of the quarter plate, two with the principal section of 

 the polarising nicol on the right, and two on the left, of the plane of 

 incidence ; the plate VI being used with red and yellow light, and plate I 

 with blue light. 



No. of film. Thickness. 



2 -0000645 millim. 



7 0-0000709 „ 



5 0-0000837 „ 



4 0-0001107 „ 



3 0-0001135 „ 



Red light. Yellow light. Blue light. 



P.I. P.A. P.I. P.A. P.I. P.A. 



O / O / o / / / o/ 



f Plate at A. 74 43 40 50 71 05 40 51 68 18 37 21 



1 Plate at B. 70 39 41 14 69 26 40 11 65 54 37 02 



L Mean value 72 41 41 02 70 15 40 31 67 06 37 11 



r Plate at A. 75 16 40 42 72 32 40 10 68 56 35 11 

 i Plate at B. 71 48 40 26 70 45 40 21 66 20 35 36 

 L Mean value 73 32 40 34 71 38 40 15 67 38 35 23 



f Plate at A. 76 44 43 27 73 59 41 45 69 51 38 30 



No. 5. \ Plate at B. 72 54 43 30 72 41 42 22 67 51 38 16 



I Mean value 74 51 43 28 73 20 42 03 68 51 38 23 



f Plate at A. 76 27 43 32 73 06 42 39 69 54 38 59 



?s T o. 4. \ Plate at B. 72 39 42 42 71 23 42 21 67 21 38 52 



L Mean value 74 33 43 07 72 14 42 30 68 37 38 55 



Plate at A. 76 34 42 56 73 20 42 02 69 49 39 09 

 1 Plate at B. 72 22 42 55 72 16 41 59 67 16 38 31 

 L Mean value 74 28 42 55 72 48 42 68 32 38 50 



Numbers 2 and 7 had been thinned with potassium cyanide, the 

 other three (3, 4, and 5, being all cut from different leaves) were of 

 their original thickness. 



The table shows that, as the thickness of the leaf increases, the 

 principal incidence and the principal azimuth increase, and to about 

 the same amount for all three kinds of light, although only portions of 

 the same leaf appear to be strictly comparable with each other. 



The values obtained with the gold leaf and with the gold plate 

 cleaned with putty powder agree fairly well together; the principal 



