164 iHr. Herschel on the Action of [March, 



order, overlapping, as represented by the dotted ovals oo, yy^ 

 &c. like the circular coloured images of the sun in the spectrum 

 of an ordinary prism, giving rise to the long prismatic tails above 

 described. Similar considerations will apply to the anomalous 

 appearances presented by the rings of all the other orders, and 

 in every situation. 



This suggests a very simple and pleasing experiment, which 

 affords an ocular demonstration of the adequacy of the explana- 

 tion I have advanced. Let a plate of Rochelle salt be placed iu 

 a tourmaline apparatus in any azimuth (45° is the most conve- 

 nient) and firmly fixed on a proper stand in a dark room. The 

 /eye being now applied, let an assistant illuminate the emeried 

 glass or lens of short focus * which disperses the light previous 

 to its incidence on the first tourmaline, with the several colours 

 .of the prismatric spectrum in succession, beginning with the red. 

 The rings will then be seen formed successively of each of the 

 colours, perfectly regular in their figure, but contracting rapidly 

 in dimension as they become illuminated with the more refran- 

 gible rays.t At the same time the pole about which they form 

 will be seen to move regularly in the direction of the other axis 

 of the crystal, and if we pass alternately from a red to a violet 

 illumination, will shift its position accordingly, backwards and 

 forwards, through a very considerable angle. If rays of two 

 colours be thrown at once on the apparatus, their two correspond- 

 ing sets of rings will be seen at once, crossing, but not obhterat- 

 ing one another, and the distance between their respective 

 centres will be observed to increase with the difference of their 

 colours. 



By measuring the interval occupied by the projections of 

 the centres of the last visible red and violet rings, as well as 

 those of the intermediate colours, on a screen at a known dist- 

 ance seen with the other eye, I found the following values of the 

 apparent and real separation of the several coloured axes : 



Between Apparent interval. Real intervaL 



Red and orange 0° 37' very uncertain .... 0° 25' 



yellow 1 50 rather uncertain . . 1 13 



green 3 43 ditto 2 29 



blue 6 5 4 3 



indigo 8 19 5 33 



violet 9 46 6 31 



As a mode of measurement this method is very inaccurate, 



• See the description of an apparatus of this kind, subjoined. 



•f- See Lectiones Optica, lib. ii. Pars. i. Obs. xiii. from which the idea of this expe- 

 riment is taken. " Magnique voluptate perfusus," says Newton, with the enthusiasm 

 of the true philosopher who loves the field he labours in, " videbam eos dilatare aut 

 contrahere se gradatim pro eo ac color luminis immutabatur." It is impossible to wit- 

 ness the very beautiful phenomenon described in the text without entering into the WBi* 

 train of feeluig. 



