202 Dr. Forster on the Variation of the reflective, 



greater southern declination, as well as to something very pe- 

 culiar in the composition of its light, we cannot get so perfect 

 a spectrum as might be desirable. This star, too, exhibits in 

 the Greatest degree a peculiar and hitherto unexplained phae- 

 nomenon, which will always interfere with our observations on 

 its permanent spectrum. I allude to the rapid permutations 

 of the colour of its light, every alternate twinkling, if I may so 

 express myself, being of an intense reddish crimson colour, and 

 the alternate one of a brilliant white. As I have before de- 

 scribed and speculated on this phsenomenon, common, though 

 in a less degree, to other stars when near to the horizon, I 

 shall not further dwell on it here, but observe that Antares, 

 considered with reference to its light, must be put among 

 anomalous stars. 



At air in the Eagle, and also the dull white stars, exhibit a 

 vast quantity of intense green light. This is very conspicuous 

 in many stars of the 2d and 3d magnitudes. 



The planets likewise present spectra very considerably dif- 

 fering from each other. Jupiter possesses all the colours ; but 

 from something in their respective proportions, or from some 

 unknown cause, this planet is liable to produce, even in good 

 and almost achromatic glasses, so bad and so coloured a spec- 

 trum, that I have always found him a disagreeable star to ob- 

 serve. As a prismatic spectrum, however, he is beautiful. The 

 green colour seems somewhat deficient in his spectrum ; never- 

 theless Jupiter appears green in comparison with Sirius when 

 an opportunity offers of viewing both at one time*. Venus 

 appears less green than Jupiter, but still she is not of so bright 

 and blueish a white as Sirius. Her spectrum in the prismatic 

 glass shows most of the rays, but the green colour is very pale. 



Saturn seems composed chiefly of the mean rays, and has a 

 very small quantity of the extreme colours. Mr. Lee, who 

 also notices this, subjoins the following judicious question — 

 Whether this may not explain why Saturn bears magnifying 

 better than Jupiter or Venus ? 



Mars, who shines with a red light, appears as a spectral 

 image on the prism to possess less of the middle and most 

 refrangible colours. The red is very conspicuous in the 

 prismatic spectrum. 



Mercury is said to show a similar spectrum : I confess I have 

 not made observations on Mercury myself. 



* We may imitate the different colours of the spectra of the several 

 stars and planets, by burning antimony, steel, and other metallic filings, in 

 pyrotechnical jerb% and viewing them through a prism. Compare the 

 prismatic spectrum of ignited steel with that of Jupiter, of burning anti- 

 mony with Sirius, of copper filings with the spectrum of Mars; and so on. 



Of 



