348 Analysis of Scientific Books and Memoirs. 



pale straw colour. In a beautifully coloured plate, or rather map, of the 

 spectrum before us, in which every part is laid down by M. Fraun- 

 hofer, from exact micrometrical measurements, the portion occupied by 

 the yellow is 22, the length of the whole spectrum being 286, and the in- 

 terval between the red and blue about 77.* Mr Herschel's estimate agrees 

 well enough with these measures. We may here take occasion to remark, that 

 although upwards of seventeen years have elapsed since the first discovery of 

 black lines in the spectrum by Wollaston, none of the continental opticians, 

 with whom we have bad opportunities of conversing, seemed aware of their 

 having been known previous to the elaborate researches of the eminent 

 artist just mentioned. 



Annexed to Mr Herschel's paper is a determination of the dispersions of 

 a variety of specimens of flint and crown glass, (by a peculiar and simple 

 method, founded on the same principle as the double image micrometer,) 

 which are considerably above the usual estimates. The dispersion assign- 

 ed by Fraunhofer are still higher, as might be expected, from the supe- 

 riority of his means of examining the spectrum at its limits. Indeed, it 

 is to his " Determination of the refractive and dispersive powers of differ- 

 ent species of glass, &c, &c," that we must refer for all accurate know- 

 ledge on this important subject. 



II. Some Account of the late M- Guinand, and of the Important Discovery 

 made by him in the Manufacture of Flint Glass for Large Telescopes. — 

 London, 1825, 25 p.f 



_L he little pamphlet, of which we propose at present to give an abstract, 

 is filled with details of the most interesting kind, both to the philosopher 

 and the general reader. 



The discovery of a method of making Flint Glass for achromatic tele- 

 scopes has been, during the last seventy years, an object of almost nation- 

 al ambition. In England, unfortunately, the strictness of our Excise 

 laws prevented any attempt from being made on a proper scale to solve 

 this great practical problem ; but in France, where no such restrictions ex- 

 isted, numerous attempts have been made to perfect the manufacture of 

 flint-glass for optical purposes. To what extent these experiments suc- 

 ceeded, we have not sufficient information to enable us to ascertain ; but 

 we believe it is universally admitted, that the difficulty was neither sur- 

 mounted, nor in the way of being surmounted, when M. Guinand, of the 

 village of Brenets, in the canton of Neufchatel, began those laborious re- 

 searches, which were finally crowned with the most complete success. 



We shall, therefore, proceed to give a brief history of the life and la- 



* An engraving of the spectrum, as laid down by Fraunhofer, with most of the 

 lines which cross it, will be found in the Edinburgh Encyclopedia, Art. Optics, 

 vol. xv. p. 548. Pl. 433 Ed. 



"f This pamphlet is a translation from an article in the Bibl'wthcquc Umvcrsclte 

 for February and March 1824. 



