Miscellanies. 209 



which may possibly invite the attention of those who are much better 

 qualified to suggest the proper course to be pursued ; and these re- 

 marks must necessarily include much that is well known to chemists 

 and opticians, that I may be clearly understood in the sequel. 



Crown glass is composed, essentially, of silex and soda ; flint glass 

 of silex, potash, and oxide of lead. The proportions of these compo- 

 nents are not very definite, and some other materials are sometimes 

 used in small quantities, for the purpose of prom.oting the fusion, or 

 correcting the accidental color resulting from the impurity of the ma- 

 terials. Crown glass has a light green color, varying from yellowish 

 to bluish ; and as the specific gravity of the difierent materials is 

 nearly equal, and their affinities considerable, it is generally quite ho- 

 mogeneous, even in large masses; but in consequence of the different 

 specific gravities of the materials of flint glass, or of their weak affini- 

 ties, or of some other cause, it is difficult to procure any considerable 

 mass of it, with the same density, and consequent refractive and dis- 

 persive powers in different parts of it. As this imperfection prubably 

 results from the natural constitution of the materials, there is but a 

 faint hope of removing it by the counteracting influence of some other 

 material in addition to those which are generally used. As the two 

 kinds of glass, crown and flint, have each a specific action on the solar 

 light, dilTeringin quality as well as in quantity, their combined action 

 in optical instruments is productive of anomalies and imperfections, 

 which, though somewhat reduced by scientific investigation and ex- 

 perimental analysis, have been but partially obviated. 



The solar spectrum produced by a crown glass prism exhibits the 

 various colors occupying certain parts of the whole length of the spec- 

 trum. The spectrum produced by a flint glass prism also exhibits the 

 various colors occupying certain parts of its whole length ; but the 

 proportions of the diflerent colored spaces are not the same in both 

 spectra; the spaces at the red end of the spectrum produced by the 

 flint glass prism being longer, and at the blue end shorter, than those 

 produced by the crown glass prism ; and the colored spaces are said 

 to be irrational. This irrationality is the cause of the greatest imper- 

 fection of a telescope well made of the best glass. Sir John Her- 

 schel has proposed to give the lenses such curvatures as will unite 

 the colors at the points where the brightest red borders on the orange, 

 and where the brightest blue borders on the green. This arrange- 

 ment will doubtless produce the best image with the glass now 

 used ; but I think it is possible to make tWo kinds of glass that will 

 have the same proportional specific action on light, as to the sev- 

 eral colors, each to each, but difierent in degree, so that the contrary 

 refractions and dispersions will unite all the colors ; and Dr. Blair, of 



Vol. XL, No. 1.— Oct.-Dec. 1840. 27 



