The Art of projection and complete magic lantern ... - Yesterday Image

58class of kns became obsolete, as it would not give the extreme sharp andcrucial definition necessary for photographs and enlargements.The formula now generally recognised and acknowledged, is that knownas the Pelzvil form or the usual portrait combination (Fig. 35) consistingof four lenses, the two front combination A. being cemented together,consisting of a plano-convex or a meniscus of slight external concavecurvature, and a double convex. The posterior lens composed of abi-convex D., and a concavo-convex B., mounted with a ring C. between,so as to prevent them touching each other. Of this class of lens, there aregood,middling, and bad, and unfortunately the system of cheapness hasbrought about a great influx of the latter class.In taking out the lenses for cleaning, it is most important that they bereplaced in their proper order, the diagram will help those taking acombination apart for tlie first time.For lenses of short focus, the back combination should be of slightlylarger diameter than that of the front. A good long focus lens is madeby taking out the back combination and using the front only, a stop isnecessary to sharpen up the sides, with this there is a great loss oflight. Naturally the same results cannot be obtained with this, as froma complete combination already described. As a rule the shorter thethe focus of the objective the greater the light on thescreen, the averageshort focus lens does not require to be of a wider diameter than 2ins.back. When a lens of longer focus than 5ins. back focus is used, thediameter must be increased to 2^ins.. When the focus is loins. andupwards, the diameter must be 3ins., nothing is gained by increasing thediameter beyond this. This is a critical point, and one which practicalknowledge alone can prove. According to theory there is no more lossof light with a long focus lens than with one of short focus, and this iswhere many writers on optics fail.The loss of light is considerable in every inch added beyond 4ins.,and the loss of light from a loin. focus lens of small diameter whencompared with a short focus, is 50 per cent., and the fact is simpleenough, when demonstrated.If a beam of light is watched coming fromthe condenser, it will then be seen that at 10 inches away a larger lensmust be substituted for the small one, if the whole of the rays are to betaken up.Compensation for the great loss of light with a long focus lens, isonly to be brought about by the use of an objective of large diameter, andthe student who can pass as much light through a loin. focus lens of