90 ANNUAL OF SCIENTIFIC DISCOVEEY. 







finally ceased, having lasted twenty-five minutes. We saw no light- 

 ning, and were puzzled at first as to whether we should be afraid or 

 amused. 



INTERESTING ELECTRIC ILLUMINATION. 



Prof. W. B. Rogers communicates to Sittiman's Journal the follow- 

 ing observations on a powerful electric illumination, exhibited in Bos- 

 ton, August, 1863, by Mr. Ritchie, the well-known electrician, as a 

 part of the display attendant on a public rejoicing. The battery in 

 question, consisting of 250 Bunsen elements, having each an acting 

 zinc surface of about eighty-five inches, and grouped in five battalions 

 of fifty each, was arranged in the dome of the State House ; and the 

 carbon light, and the photometric apparatus prepared for the purpose 

 were placed in line across the same apartment, commanding a range 

 of fifty feet. Prof. Rogers says : 



In view of the immense power of the light, as observed in the pre- 

 vious experiment, I substituted for the 20-candle gas burner, used at 

 that time as the standard of comparison, a unit ten times as great, 

 formed by the flame of a kerosene lamp placed in the focus of a small 

 parabolic reflector, and throwing its concentrated light on a photomet- 

 ric screen of prepared paper fixed in front of it at the distance of five 

 feet. Before the observation, the lamp and reflector were so adjusted 

 as to make the light cast on the near side of the screen equivalent by 

 measure to the action of 200 candles. 



This was done by the intervention of a kerosene lamp fitted up with 

 a bridge of platinum wire for defining and restricting the height of the 

 square flame. Such a lamp I find of frequent use in ordinary photome- 

 try, as, when suitably adjusted, it gives t^e light of about eight stand- 

 ard candles, and thus transfers the measurement in the photometer to 

 the wider divisions of the scale. Being suspended in a balance of pe- 

 culiar construction, its rate of consumption enables us to correct for 

 any slight departure from the assigned illumination. The lamp thus 

 regulated was placed with its flat flame twelve inches from the screen, 

 while the lamp in the reflector was distant sixty inches, and the flame 

 of the latter was adjusted until the effects on the screen were equalized. 



A platform supporting the standard lamp and screen at the assigned 

 distance was arranged to slide on a horizontal graduated bar, extend- 

 ing directly toward the carbon points so that the screen should receive 

 the rays from the electric light and from the reflector perpendiculaiiy 

 on its opposite faces, In making the observations, the platform was 

 moved to and fro until the illumination on the opposite sides of the 

 screen was judged to be equal, and then the measured distances of the 

 two antagonizing lights from the screen gave by easy computation their 

 relative illuminating power. 



By a series of such observations, it was found that the carbon light 

 had a force varying from 52 to 61 times that of the lamp with reflector, 

 making it equivalent in illuminating power to the action of from 10,000 

 to 12,000 standard sperm candles, pouring their light from the same 

 distance upon the surface of the screen. This, it will l3e remembered, is 

 the effect of the unaided carbon-light sending its rays equally in all 

 directions from the luminous centre, and falls vastly short of the illumin- 

 ating force of the cone of collected rays which was seen stretching, 



