32 NEW MANUFACTURES REQUIRING 



By these means Mr. Faraday has prepared homogeneous 

 plates of heavy optical glass seven inches square and eight 

 pounds in weight; he can thus at pleasure obtain, and he 

 has placed it within the power of others to obtain, with equal 

 certainty, a glass perfectly free from stria!, unexceptionable 

 in hardness, and having less colour than the crown-glass con- 

 stantly used in the construction of telescopes : " but it is," he 

 observes, " the simultaneous absence of all striae and bubbles, 

 with at the same time that degree of hardness and colour which 

 will render the glass fit for optical purposes, that I am aiming 

 at, and that I trust shortly to obtain*." 



Unlike the cases before recited of the important benefits 

 conferred upon the cultivators of the useful arts by scientific 

 knowledge, the manufacture of the new species of glass is not 

 yet complete. Glass has been made, and telescopes have been 

 manufactured with it; still greater improvements however will 

 undoubtedly be effected ; and much more time must be ex- 

 pected to elapse before the investigation can be considered as 

 finished ; but a decided step has been gained, directly by the 

 resources of science, in the manufacture of glass for optical 

 purposes. 



In this case it may be observed, as in that of the invention 

 of Mr. Barlow's Correcting- Plate, that what was beyond the 

 power of the arts and their professors to effect, has been ac- 



to purify the oxide of lead by converting litharge into a nitrate, and subject- 

 ing it repeatedly to crystallization. This salt was then used in making the 

 glass ; 166 parts being used for every 112 parts of oxide required to form 

 the glass. 



* Nearly all the information contained in the above sketch of the recent 

 improvement in the manufacture of glass for optical purposes, has been de- 

 rived from Mr. Faraday's Bakerian Lecture on that subject, published in the 

 Philosophical Transactions of the Royal Society for 1830, Part I. In this 

 paper are given full and efficient instructions to those who may desire to 

 manufacture optical glass. The inquiry, it will be remembered, was under- 

 taken solely for the interests of science, which are the interests, in a certain 

 respect, of mankind at large, and its results, so far as it has yet proceeded, 

 have been made public accordingly. In agreement with this design, it is 

 stated in the Bakerian Lecture, that down to the period of its communication 

 to the Royal Society, " The attention [of Mr. Faraday and his colleagues] 

 has been devoted, as it still must be for a while, to the establishment of a 

 process, which, competent to produce with certainty a glass fitted for opti- 

 cal purposes, may have the philosophy and practice of every part so fully 

 ascertained, as to be capable of description in a manner sufficiently clear to 

 enable any other person, with moderate care, to obtain the same results 

 without the labour of long and tedious investigation.'' 



But although the information above given has been derived from Mr. Fa- 

 raday's Bakerian Lecture, it has been so materially altered by abbreviation* 

 and in arrangement, that that gentleman must not be considered accountable 

 for any statements 1 have given, but those which are distinguished by quo- 

 tation marks, unless they shall be verified by reference to the Bukerian 

 Lecture. 



