of the Atmosphere, and on the Peculiarities of Stars. 335 



Vibration producing the primitive Colours. 



I have noticed a most remarkable method of separating the 

 light of certain stars by means of rapidly altering the position 

 ot the glass. In order the more easily to explain this method, 

 I must be a little prolix. The experiment of producing a 

 ring of light like a circular band or wheel, by means of twirl- 

 ing round a piece of lighted wood tied to a string, is well 

 known, and is commonly practised by children. Now by 

 causing the telescope to gyrate in such a manner, on the 

 swivel on which it is mounted capable of being moved in all 

 directions, as to cause the star viewed at the time to describe 

 a circle ; we obtain a luminous ring in the field of view of the 

 telescope, instead of a luminous point. Now there is nothing 

 very remarkable in this optical experiment, as far as it goes ; 

 for we can produce various figures very easily, according to 

 the direction in which we move the telescope at the time we 

 are observing a star. Thus, if we move it rapidly from one 

 side to the other horizontally, instead of seeing a luminous 

 point, the star will appear like a horizontal line of light. If 

 we gyrate it round and round, we shall view a circle of light ; 

 and so on. This is easily understood, and the effect is com- 

 monly referred to deceptio visus. But I have now to intro- 

 duce to notice a very extraordinary phenomenon connected 

 with this simple experiment. Observing Lyra one evening, 

 while I gave the telescope the gyrating motion described, I 

 noticed that the ring of light produced was not uniform in 

 colour, as is the case when we twirl round a blue candle or a 

 piece of lighted wood on a string. The ring of light pro- 

 duced by the rapid apparent gyration of Lyra in the glass 

 was separated into the prismatic colours, each colour seeming 

 to occupy a certain portion of the circular ring of light. 

 Now by observing which colour was most intense and occu- 

 pied the largest portion of the ring, I endeavoured to ascertain 

 the relative proportions of the primitive rays, as these co- 

 loured portions of the ring seemed to correspond to the 

 primitive colour of which the star is composed, and which 

 we see when the light is separated by the prism adapted to 

 the telescope. Thus the blue was the most conspicuous in 

 Lyra : for though the successive portions of the ring were 

 red, yellow, green ami indigo; yet all these colours were 

 weak compared with the blue. I should have thought the 

 whole of this phenomenon of no moment, had not the co- 

 lours thus produced varied in the case of different stars. I 

 tried the experiment immediately on the planet Mars, then 

 favourably situated for observation ; and I found that the cir- 

 cular 



