46 Professors Ayrton and Perry on a 



50 or 100 feet away, which was the plan in use until that time. 

 We exhibit now five successive forms of the instrument, which 

 illustrate the history of its development to the present time. 



1. The first of these is very nearly the same as that described 

 in our former paper, with the exception that we discarded the 

 use of a long screw (shown in our original figure) for adjusting 

 the position of the lens — as we found that a very easy adjust- 

 ment might be effected with the fingers, the tension of the 

 bellows part making an automatic clutch which fixed the lens- 

 slide in any position. 



2. The second specimen is on the same principle, only that 

 telescope-tubes are used instead of a wooden frame and a bel- 

 lows. Instead of the lens part alone tilting when the elevated 

 or depressed light has to be examined, the candle-box is here 

 made to tilt also, the candle being supported in gimbals so that 

 it may remain vertical for every angle of elevation. 



3. The third specimen is on pretty much the same principle; 

 but as we found a difficulty in comparing two illuminated 

 disks whose centres were some distance apart, we arranged in 

 front of these disks two mirrors, which enable us to make the 

 comparison between two illuminated semicircles having the 

 same diameter. The difficulty of adjusting the lens and 

 making a comparison of the illuminations, and reading the 

 scale, without moving one's head, in all these early instru- 

 ments led us to the 



4th form, which is probably familiar to the Members, as it 

 was exhibited at Paris and largely used there for measure- 

 ments. In this the candle-box and the lens-box are placed 

 end to end, the lens is fixed in a wooden piston which moves 

 in its hollow square box, which is lined with velvet; and the 

 lens shows its position by a pointer moving over the scale 

 outside. The pointer projects from the inside of the wooden 

 cylinder at any point of a long slot, whose sides are made of 

 india-rubber tubing, so that no extraneous light can reach the 

 illuminated screen. A little handle working a rack and pinion 

 enables the lens to be placed in any position. Through a hole 

 at the side the two screens can be viewed reflected in two 

 mirrors, inclined to one another in the space between the 

 candle-box and the lens-cylinder; and the illuminated papers 

 are viewed as two semicircles having a common diameter. In 

 front of this hole we have slides of red and green glass ; so that, 

 as our custom has always been, we make two measurements — 

 one a comparison of the ruby-red light of the lamp examined 

 with the red light of the candle, and another of the green 

 lights. This instrument differed from the earlier forms in not 

 requiring any calculation to be made of the strength of the 



