400 Aslronomkal and Nautical Collections. 



2. The second memoir appears to exhibit a less fortunate at- 

 tempt to obtain novelty without essential improvement. It re- 

 lates to " the construction of an achromatic telescope without 

 lenses, and with a single refractive medium." " It has hitherto 

 been believed," says the author, " by natural philosophers, that 

 the dispersion of colours is constant for the same refracting me- 

 dium, or that a given refraction, produced by the same sub- 

 stance, is accompanied by a given dispersion. But I have 

 found that the dispersion produced by more than one refraction 

 is not by any means constant, but varies according to the va- 

 rious inclinations of the incident ray." He finds, however, that 

 this property is really deducible from the constant proportion of 

 the sines, and observes in conclusion, that " although the theory 

 of colours has been cultivated by so many distinguished mathe- 

 maticians and opticians, from the days of Newton to the present 

 time, the property here described not only remained unknown, 

 but the thing was judged impossible, until I discovered its prac- 

 ticability by means of some experiments which I was making 

 with another view. We may therefore consider this circum- 

 stance as a striking proof, among many others, that in the pro- 

 secution of physical science, experiment is very often, and per- 

 haps most commonly, more successful than theory, witn regard 

 to the development of all the circumstances that accompany a 

 given phenomenon." 



Now it is well known, that Euler was aware of the difference 

 of dispersion that might be obtained in this manner from the 

 same refractive substance, and the author himself quotes the 

 work of our countryman. Dr. Brewster, who has entered very 

 fully into the investigation of the subject. " The celebrated Dr. 

 Brewster, " he says, " in his excellent Treatise on New Instru- 

 ments, informs us, page 400, that he has made several attempts 

 to exclude colours by me^ns of an object glass composed of two 

 lenses of the same substance ; but his experiments were not 

 crowned with the desired success." 



Mr. Amici, however, appears to have been considerably more 

 successful in a practical point of view. He informs us that 

 " ever since the year 1815, he has made telescopes of prisms of 

 larger and smaller angles, which have fully answered his ex- 



