630 



REPORT— 1900. 



right-liaud square, and that of the right-hand prating on tlie left-hand square, the 

 two spectra, which can be adjusted to the same intensity, being thus seen side by 

 side, one with the left eye on a red ground, and the otlier with the right eye on a 

 blue ground. The red glass produces partial red blindness of the lefc eye, and the 

 spectrum seen by it lacks red, the other colours being unaltered. And for a similar 

 reason the spectrum seen by the right eye lacks blue, the effect being more notice- 

 able owing to the contrast of sensation in the two eyes. In the author's opinion 

 this experiment nffords further confirmation of the views of Scherffer, Darwin, and 

 Youns: in regard to contrast. 



6. A Quartz-Calcite Symmetrical Doublet. By J. W. Gifford. 



The difficulty in constructing lenses of crystals consists chiefly in the double 

 refraction, whicti causes confusion. As quartz is a positive, and calcite a negative, 

 crystal, they tend to correct one another, altliough the separation of the lines in 

 quartz is only one-twentietli of that in calcite. Both lenses are cut with their 

 flxes corresponding to the axes of the crystals. The wave-length situated at the 

 point of greatest actinic activity is about 2748, as found by averaging the position 

 of bright lines of the principal spectra as follows : 



W. L. Centre 

 of maximum effect 

 . 3051 

 . 2571 

 . 2444 

 . 2465 



2760-6 = Average. 



This was equalised with "\Y. L. 5G07 or the centre of visual activity. The 

 indices were determined by using prisms polished on three sides, and by averaging 

 the observations, so that the angle of the prism might be taken as exactly 60. 

 The temperature was 69° Fahrenheit. 



In calculating the radii the formula W 

 results for unity : 



was used, with the following 



