APPARA TUS FOR SCIENTIFIC DEMONSTRA TION 173 



lime-light greater brilliance is often desirable, especially with 

 such a projection as a Barton's button or a Lissajous' com- 

 bination figure. Even a polariscope, with field of, say, If 

 diameter, is improved in performance by sending all the light 

 from the condensers through such a field. This is easily 

 managed by a reducing system of lenses similar to that 

 shown in fig. 95, but the convex and concave lenses being of 

 greater foci and diameter, and so adjusted (with a movable 

 adjustment is desirable) that when the radiant and condensers 

 are arranged for a parallel beam, and the system inserted 

 in the flange-nozzle, the rays converged by the convex lens are 

 re-parallelised by the concave within two inches diameter. A 

 parallel beam of two inches 

 is convenient for many '""" 

 purposes, besides giving 

 greater brilliance to polar- 

 iscope projections. 



But still more useful 

 is a smaller arrangement 

 of the same kind, in which 

 the condensing lens is of 



about IT, inches diameter and 2 inches focus, and the concave 

 paralleliser about f inch diameter, with removable caps or 

 apertures from about 2 mm. to 16 mm. fitting in the front, as 

 in fig. 95. The light from the condensers is slightly con- 

 verged upon the back lens, and by this further converged upon 

 the concave, where it is parallelised as a small pencil. By such 

 an attachment a very brilliant pencil of light of any diameter 

 from 16 mm. downwards can be obtained with the 0. H. jet. 

 The rays will of course scatter or diverge considerably, even 

 when parallelised as far as possible ; but in acoustical pro- 

 jections this is an advantage rather than otherwise. 



Such a ' pencil ' attachment has another use. By removing 

 the concave paralleliser (which should be removable) a strong 

 conical pencil can be thrown through a very small hole, in 



FIG. 95. Pencil Attachment 



