. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. ity to the two doubleconvex lenses, c, fig. 10. Butit will be evident that the othercondition, of minimum devia-tion at the margin, is only ap-proximated to when the curvesor thicknesses of the lenses arein some proportion to the foci oneach side of the condenser ( position of the radiant, andthe position of its image on the other side of the condenser).Hence, for a lime-light condenser, the lens next the radiantshould be of considerably deeper curve, the two lenses takingthe form of
. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. ity to the two doubleconvex lenses, c, fig. 10. Butit will be evident that the othercondition, of minimum devia-tion at the margin, is only ap-proximated to when the curvesor thicknesses of the lenses arein some proportion to the foci oneach side of the condenser ( position of the radiant, andthe position of its image on the other side of the condenser).Hence, for a lime-light condenser, the lens next the radiantshould be of considerably deeper curve, the two lenses takingthe form of fig. 12 rather than of e in fig. 10. Then thespherical aberration f/ will also be comparatively thicker lens, however, is more in danger of cracking fromthe heat; therefore, as it will be obvious that a somewhatsmaller diameter at d d will collect all the bundle of divergingrays which can reach the second lens, d, this fact shouldbe taken advantage of, in order to reduce its thickness whilekeeping the deeper curve (see fig. 13). All things considered, I regard this as practically the best. THE PARTS OF A LANTERN 21 model for a lantern condenser. Some opticians—includingMr. Dallmeyer—prefer to make the lens next the slide a1 crossed lens ; but practically there is no advantage in this,and it (slightly) diminishes the size of the slide which a givencondenser will cover, as already indicated. There remains to consider the chromatic aberration; forof course the margin of a lens acts precisely like a prism,and makes the different colours diverge into a spectrum, as iseasily seen by experiment. Now these diverging rays fromthe first lens, falling on the second lens, are by it more or lessconverged, like any other rays diverging at the same amount of this convergence is greater, the farther therays are allowed to proceed before convergence by the secondlens ; the conjugate foci are altered in relation to each other, asin other cases. Hence there is one particul
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