The Philosophical magazine; a journal of theoretical, experimental and applied physics . this subject. Instead of employing only one species of glass as recom-mended by Dr. Wollaston, I make use of several, each ofwhich is suited to the nature of ihe substance under examina-tion, and that species of glass is selected for the purpose, which,when the substance to be examined is applied to the base ofthe prism, gives with the subject so applied, when viewed inits position, neither too acute or too obtuse an incidence; forit is at extreme incidences, as far as I have been able to ob-serve by the m
The Philosophical magazine; a journal of theoretical, experimental and applied physics . this subject. Instead of employing only one species of glass as recom-mended by Dr. Wollaston, I make use of several, each ofwhich is suited to the nature of ihe substance under examina-tion, and that species of glass is selected for the purpose, which,when the substance to be examined is applied to the base ofthe prism, gives with the subject so applied, when viewed inits position, neither too acute or too obtuse an incidence; forit is at extreme incidences, as far as I have been able to ob-serve by the means here employed, that erroneous results areliable to be obtained. The glass prisms which I am in thehabit of using have respectively a refractive index for Fraun-hofers ray b of 1*516, which is ordinary plate glass. 1583, which is common flint glass. 1635, a very heavy flint glass, which I made some yearsago for optical purposes. 1*816, Faradays borate of lead glass. Now in order that these different prisms may be used, amodification of the apparatus as originally proposed by g d e Wollaston is requisite, or otherwise each piism will rec|uire aseparate and distinct instrument; but to accomplish this ob-ject with but one instrument, I have changed the position ofthe indicator from the longer of the bars to the shorter, andconstructed the loufjest bar a in such a manner that it is ca-pable of being extended in length from 15-lG to 1816 niches, Listnunent for ascertaining Refracting Indices, 8)-c. 511 while the height of the shorter bar h retains the ori^nal lengthproposed by Dr. Wollaston of ten inches, and the indicator c,as a consequence, has only the half of this lengtii, that is tosay five inches. The bottom bar d is about two feet in length,and has a dove-tailed groove or furrow ploughed in it through-out its whole length ; in this groove a piece e is made to slideeasily, which may be clamped or made last by means of athumb-screw in any part that may be required. To
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