AMERICAN INSTITUTE. 419 



never could tell. "VVe put the flint glass inside and the crown out, because 

 the flint glass is liable to injury, being much weaker — in fact a wafer put 

 on it, with paper over it, will when dry pull ofi" some of the glass with it. 

 Rock crystal is best if we could have that throne of a single one four feet 

 thick, mentioned by Bayard Taylor, we could saw out an object glass of 

 great power. You all know that it takes two kinds of glass to make one. 

 He described the experiments of Newton and others on colors of the rays 

 of light, their various reflexibility and refrangibility, and the fact that the 

 red ray passes more directly through any translucent medium, and the ex- 

 periments made to get rid of the prismatic colors entirely, so as to form 

 lenses perfectly achromatic, [a-ckroma, without color). Dolland, a silk 

 weaver, made by circumstances an astronomical instrument maker, discov- 

 ered that the bottoms of green and white glass tumblers together gave color- 

 less light, and he made an object glass of the two — crown and flint, ground 

 to fit each other so exactly as to appear but one to the eye — it proved achro- 

 matic and remains still the best refractor. You all know the great reflec- 

 tors ; Lord Rosse's, weighs about three tons. All these solid mirrors are 

 difiicult to manage in celestial observations, on account of the serious diffi- 

 culty of difi'erence of temperature between them and air, for when air is 

 warmer the moisture is immediately condensed on the mirror, so that very 

 often the observer becomes disappointed, wipes it clean, and the polish of 

 it sufi"ers. He described the proceeding relative to the manufacture of the 

 object glass in England. A mathematician first detenuined its figure by 

 angular investigations — the practical grinder follows them, &c. I grind 

 my object glasses without any such trouble. My largest object glass, 12f 

 inches, is at the Ann Arbour Observatory, on a rising ground, where the 

 winds from the prairies are felt in the observatory. I can make such a 

 telescope for $6,750. 



Mr. Meigs, — I had a correspondence twenty years ago with Messrs. 

 rrauenhoff"er and Utzschneider, relative to one of their refractors for New 

 'York — ten inch aperture. They required ten thousand dollars, to be de- 

 posited in Europe for it. It embraced equatorial movement. 



Mr. Fitz in reply to a question of Mr. Meigs, said he preferred the refrac- 

 tors to the reflectors, even those of only four inch aperture. As to the 

 intense heat required in melting the glass, the potash in it makes it neces- 

 sary to its vitrification. 



]Mr. Butler. — Why will not platinum, covered with burned clay, make a 

 good stirrer of the melted glass ? 



Rev. Mr. Adamson. — Was at the the Herschel Observatory, at Good 

 Hope, and tried the power of Sir John's telescope. Although the air is 

 clear there, yet doubtless more perfect view would be at elevated position, 

 even a mile and an half high, above general cloud, &c., more perfect defini- 

 tion of objects can be obtained, 



Mr. Fitz. — A dull atmosphere, yet clear, is best for a very clear night 

 when the stars seem peculiarly sparkling, causes the object to dance, I may 

 say. 



