. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. he jet be kept in work while the lantern remains the jet is turned off so that the flame of coal-gas, releasedfrom the pressure driving it down, rises upwards towards thecondenser, a crack will probably result; but it is easilyavoided with this caution, and more brilliant projections arethus obtained than in the other way. 86. Erecting Prism.—For the mass of physical experimentsthe ordinary inversion of an image does not signify, and is readily understood by theaudience. For ther
. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. he jet be kept in work while the lantern remains the jet is turned off so that the flame of coal-gas, releasedfrom the pressure driving it down, rises upwards towards thecondenser, a crack will probably result; but it is easilyavoided with this caution, and more brilliant projections arethus obtained than in the other way. 86. Erecting Prism.—For the mass of physical experimentsthe ordinary inversion of an image does not signify, and is readily understood by theaudience. For thermometricand some other experiments,j* however, it becomes of im-£ portance, and in such casesis obviated by the use of aprism, as described by Bertin,Miiller, and others. Fig. 93shows the action of a right-angled prism p used in this way. The ray a b is refractedto c and there reflected, and again refracted as d e; whilefg is refracted to h and there reflected, coming out as i k ;thus the top and bottom rays are inverted, and the image iserected. The best position for the prism is where the cone. Fig. 93 —Erecting Prism APPARA TUS FOR SCIENTIFIC DEMONSTRA TION 171 of rays from the lens is reduced to its smallest diameter atthe apparent crossing-point. A right-angled prism is ordinarily employed, but is not thebest form, as a large portion of the prism at the apex p isquite outside the field of all inverted rays. By making theprism with all this waste portion truncated, the same effectivefield can be obtained with a much less massive piece ofoptical glass, and the cost thereby much reduced. Anothermethod is to employ a greater angle than 90°, and Zentmayeiadopted an angle of 126°, which utilises all the field to theapex. Such a large angle, however, loses appreciably morelight by reflection; and, on the whole, I consider the bestresults are obtained by a combination of the two methods,adopting an angle of about 105° and truncating the prismsufficiently to have no surplus
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Keywords: ., bookauthorwrightle, bookcentury1900, bookdecade1900, bookyear1906
