172 OPTICAL PROJECTION 



spring M, and worked by the hand through two studs working 

 in slots at the top and bottom of B c. Into another collar at 

 c screws the jacket D of the focussing tube, which may either 

 slide nicely, or have a rack and pinion E. The lens tube F is 

 about 2 inches diameter, and carries a combination power of 

 two lenses, the back one a plano-convex 2 niches diameter, 

 plane side towards the stage, and 5 or 6 inches focus (both are 

 convenient), and the front lens rather over 1^ diameter, about 

 8 inches focus. This lens H screws into a collar from which 

 projects a nozzle, N, If external diameter. The exact diameters 

 are, of course, not material. A rack and pinion has advantages 

 and disadvantages. The advantages are obvious. On the 

 other hand, a sliding movement may be made very pleasant if 

 cloth-lined, has more range, and allows of easy withdrawal of 

 the power, and substitution of a slit or series of apertures. 

 With a rack front this substitution has to be effected by un- 

 screwing the jacket from c, and substituting a plain jacket to 

 carry those fittings. 



Such a front is very useful for a great deal of work in 

 optics and acoustics. Though the lenses are simple, it is free 

 enough from aberrations for practical purposes, and is the best 

 power for a polariscope. On its smaller slides a great deal 

 more light can be condensed ; or a slit for spectrum work, or 

 an aperture for diffraction, can have sufficient light condensed 

 upon it to work 'direct' for many experiments, without any 

 other lenses. Finally, when the power is removed, and a slit 

 or aperture placed in the front of the jacket, by inserting in 

 the stage E a cylindrical or spherical lens mounted like a slide 

 in a wooden frame 4 x 2J inches, the aperture or slit can be 

 easily adjusted in the focus of the lens, and the utmost amount 

 of light thus brought to bear upon the phenomena. 



88. Parallel Pencils. With the arc-light as a radiant, no 

 further arrangement is required for small parallel pencils 

 than to pass a parallel beam from the condensers through the 

 most appropriate hole in a circle of apertures. But with the 



