390 Intelligence and Miscellaneous Articles. 



allow me to apply this instrument to direct and precise cyanometric 

 observations. 



To convert this apparatus into a cyanoraeter, the plate with 

 double rotation is removed ; a Nicol's prism is substituted for the 

 doubly refracting prism ; a jointed screen, consisting of a frame upon 

 which a sheet of white paper is stretched, is fitted to the objective 

 extremity of the tube ; between the two Nicol's prisms a plate of 

 quartz of 1 millira. in thickness is placed perpendicularly to the 

 axis and a thin plate parallel to the axis, giving a violet tint, and 

 its complementary colour in polarized light. The first of these may 

 be taken out of the apparatus by a lateral aperture into which the 

 piece sujjporting it slides ; the second, fixed at one extremity of an 

 alidade, is placed at the centre of the circle of polarization in a 

 grooved piece ; the rotation of this plate, independent of that of the 

 analyser, is nevertheless measured upon the same circle by means of 

 a vernier supported upon the other extremity of the alidade. This 

 arrangement also allows the plate to be removed to regulate the 

 instrument. 



The white light reflected by the screen, after undergoing the 

 action of the crystalline plates, acquires a tint which depends upon 

 the inclination of the axis of the thin j)late to the plane of the 

 principal section of the polarizer, and upon the azimuth of the 

 principal section of the analyser. 



This system, as we shall see hereafter, allows the tint of any part 

 of the sky to be reproduced with facility ; but to operate with 

 certainty, it is necessary that the intensity of the latter should be 

 brought to the intensity of that with which it is compared ; for this 

 purpose a second tube, furnished with two Nicol's prisms, is fitted by 

 grinding into a collar screwed to the first portion of the instrument 

 parallel to the first ; the rays which have traversed these two systems 

 are completely reflected at a right angle upon the hypothenuses of 

 two isosceles right-angled prisms, by means of which the images, 

 being brought together until they almost touch, may be examined 

 at a single glance. The instrument being directed to the part of 

 the sky to be observed, we try which is the azimuth of the analyser 

 of the system for reproducing the tint, for which this tint approaches 

 that of the other image most closely ; the latter is then brought to 

 an intensity exactly equal to the former by a suitable rotation of the 

 corresponding analyser; the diff'erence of tint, if it exists, is thus 

 more apparent, and may be corrected either by varying the azimuth 

 of the analyser of the other system, or by slightly changing the 

 inclination of the thin plate, so as to compensate the excess of green 

 or violet light which occurs in the primitive tint. This comparison 

 is very easily made with a little practice. 



By means of these two angles furnished by observation and the 

 known thickness of the two plates of quartz, we may determine 

 what I shall call the cyanometric state of a point of the sky, that is 

 to say, the relation of the quantity of blue light to that of white 

 light, the mixture of which would produce the tint observed by the 

 eye. From the thickness of the plate of quartz, we deduce, as has 

 been shown by M. Biot, the rotation of the planes of polarization 



