Some Physical Properties of Nitric Acid Solutions. 109 



IV. Parallelism of Quartz Plate*. 



This was ascertained (<i) by an optical method, (b) by a spher- 

 ometer reading to O'OOl mm. In the first method the two plates 

 A and B were placed successively in a vertical position on the prism 

 table of the spectrometer, which was turned until one edge of the 

 image of the reflected slit of the collimator coincided with the inter- 

 section of the cross threads; micrometer readings were then taken. 

 The divided circle was then rotated until the image of the same portion 

 of the slit was seen reflected from the other face of the plate, and a 

 second set of readings taken. The whole series of operations was 

 repeated several times and with different parts of the circle. By this 

 method the faces of one plate A were found to deviate from true 

 parallelism by 12" of arc, while those of the other plate B were optically 

 parallel. 



In the second method the thickness of each plate was determined at 

 four points, each about 2'5 cm. from the four angles along the 

 diagonals ; the results were as under : 



Plate A. Plate E. 



6 -056 mm. 6 '044 mm. 



049 -043 



04^> -045 



vt-i ,, v*-j . , 



049 -043 



Mean... 6'049 Mean... 6 '044 



Both of these methods show that no sensible error could arise from 

 imperfect parallelism. 



V. Angle of Prism. 



The refractive indices of twenty-four samples were determined with 

 a refractive angle of prism of 60 a 8' 29" (mean value of numerous con- 

 cordant measurements), the remaining sixteen samples with a refrac- 

 tive angle of 60 6' 7" (mean value also of concordant measure- 

 ments) ; in every case the angle was measured by the usual method of 

 keeping the telescope of the spectrometer in a fixed position and 

 rotating the prism. 



It is possible that the former and higher value arises from some 

 fragment of dust or extraneous matter between one of the quartz 

 blocks and body of prism, and the supposition is apparently borne out 

 by the fact that though the prism has been subsequently re-built on 

 frequent occasions, yet the latter and lower value has invariably been 

 obtained. 



