Scientific Lectures. 53 



this should take place all over the sky, that stars should be seen two 

 by two, that would not seem to be the result of the merest colloca- 

 tion. Accordingly, sometime before he died, Sir William Herschel, 

 the trumpeter in the Hungarian Guards, who afterwards " wrote his 

 name in the heavens," as has been said, the illustrious astronomer and 

 remarkable as a man — noticed this curious aiTangement. He also 

 began to notice whether the times of stars were changed, hoping that 

 generations after him would make out something with regard to them. 

 In forty-six years, or so, one of these circuits was accomplished. So 

 in a great many other instances. They turn around one another, or 

 rather around a common point between them, sun around sun, and 

 form a constellation like a planet and its satellites. We can see very 

 much of them, but the spectroscope is going to tell more, probably. 

 What is very, very beautiful, is that they appear colored, often. 

 A star, which seen by the bare eye is white, when scrutinized 

 with the telescope is made of rose color, or green. One of the 

 most beautiful objects in the heavens is two stars, one yellow, the 

 other blue ; but to the naked eye, by a proper mingling of the colors, 

 they all appear white. Oh, why this beautiful arrangement ? I can 

 only ask the question at the present time. The greatest poetic 

 imagination almost ever exercised, that of Milton, never conceived 

 anything more sublimely magnificent in the instruction of the Angel 

 to Adam than is found in these dainty scenes, these parti-colored 

 gems in the coronet that surrounds the dark brow of night. J^ow 

 this comixture of colors makes white light. And here comes in a 

 difficulty, and a neat artistic mode of overcoming it in the telescope 

 I have spoken of. When the light passes through the edge of the 

 object glass, the white light is separated into seven different colors, 

 more or less. If, however, you take a second glass, concave or con- 

 vex, and made of a heavier and denser glass, you need not make 

 tliis one undo the convergency of the other ; it will only partially 

 prevent the rays of light coming together, not let them come together 

 so soon ; but it will take hold of the colored light, the light which 

 was the most bent, the violet, it will throw it back upon the light 

 which was the least bent, and thus the angle of all will produce 

 white light. That is the colorless, the acromatic prism introduced 

 by Peter Dollond about the middle of the last century. Now, to pro- 

 duce a combined object-glass which wall be exactly that, is a matter 

 of far greater difficulty and expense than you can possibly, at first, 

 judge of properly. In the first place, there must be two pieces of 

 gl-ass, each as pure as the clearest jelly, so you can almost look 



