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The Optical Magic Lantern Journal and Photographic Enlarger.
123
In Fig. III. the light is represented at twelve inches away, which elongates the anterior conjugate focus of the projected rays.
When the illuminant is brought closer to the |
lens than its solar focus, the rays, after passing through the lens, no longer meet, but are spread apart, as shown in Figs. [V. and V.
aos ~B \ XN — q N ~ Fig TV SA Py 4 raf ¢ ’ , a / a ne
It will be observed that in every case the
angle a Bc is the same, so that it may appear a | simple matter to calculate at what distance the |
illuminant should be placed from the lens, in order to project the desired size of disc on the object to be illuminated. ;
But now another factor has to be considered. All single lenses give what is known as spherical aberration, that is to say, rays falling at the sides of a spherical lens are not bent or refracted at the same angle as those falling more toward the central portion of the lens ; the more the curvature, or in other words the shorter the focus of the lens, the
greater is the spherical aberration. Fig. VI. = -~_ 4+ 4 os 2 __ ee eee
explains this with regard to parallel rays falling upon the lens, and shows about where the various rays come to a focus, whereas Fig. VII. will give an idea as to the direction of the rays from a given point. It will thus be apparent that in order to illuminate the stated surface at the distance required by our correspondent, a lens of very large diameter would be requisite by reason of the outer sides being rendered
‘ towards those
ineffective owing to the rays from them bending emanating from the more central portions of the condenser. How then, it may be asked, can this difficulty be overcome ? The reply is, by using a condenser composed of two lenses, so that when mounted together as in Fig. VIII., they possess the same focus as
FIGVE
that composed of the one spherical condenser. This too will possess spherical aberration, but in a much less degree than the former; and in order to make due allowance, a condenser to do the work mentioned at the beginning of this article will require to be one of about six inches in diameter and eight or nine inches combined focus. A still simpler method would be to use an ordinary magic lantern with an objective of four or four-and-a-half inch focus, this would cover the desired space; but the simplest plan of all would be to use the naked limelight behind a plate of tin in which a hole has been cut, and adjust the distance of the light behind it until the desired spread of illumination is obtained.
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Art in relation to Lantern Slides.
Now is the time of year when one can saunter down the country lanes with his tripod over his shoulder, admiring the beauties of Nature. Casting his eye to the right of him, he can behold the dim steeple in the distance, bathed in that atmospheric effect so much admired of late, whilst on his left he beholds the rural windmill, the arms of which rotate according to the dictates of the wind. Being thus placed, he naturally thinks of the fine effect to be obtained in a negative of the surroundings. Going still further along the road, he sees the wayside ponds which contain life in a multitude of forms, the contemplation of which recalls many a... . The above is a fair sample of twenty pages of an