364 RECORD OF SCIENCE FOR 1886. 



glass plate placed behind the prism and toward the eye piece, this plate 

 being illuminated by the light to be measured. When the lights are 

 of the same color, the measurements with this instrument are very exact, 

 but if not, this precision could not be obtained. By placing a red, a 

 green, and then a blue glass before the eye-piece and taking the mean, 

 a closely approximate result may be obtained with this photometer. 

 (Nature, March, 1S86, xxxill, 480.) 



Subsequently Koenig described to the society a photometer sent to 

 him from Dublin, which apparently far surpassed the Bunsen form of 

 instrument. It consisted of two quadratic prisms of cast paraffin, con- 

 nected to each other along one side. Between these two prisms was 

 placed a piece of silver or of tin foil. When light fell on one of the 

 prisms, this prism appeared clear white on account of the diffused re- 

 flections. This light was able to reach the other prism only through 

 the metallic foil, and it therefore appeared dark. But when a second 

 source of light was placed on the other side, the second prism appeared 

 also bright. By displacement along a graduated scale the photometer 

 could be brought into the position where both prisms appeared equally 

 bright. A reading of the distances-enabled the ratio of the intensities 

 to be readily calculated. (Nature, May, 1886, xxxiv, 48.) 



The Bakerian lecture before the Royal Society was on color photome- 

 try, and gave the n^sults of Captain Abney's and Major-General Fest- 

 ing's researches in the direction of ascertaining whether it was practi- 

 cable to compare with each other the intensity of lights of difl'erent 

 colors. They found that by placing a rod in front of a patch of mono- 

 chromatic light thrown on a screen, and by casting another shadow by the 

 side of the first by means of a candle, the intensities of the two lights 

 which illuminated the two shadows could be compared by what they 

 term an oscillation method. As to the value of mixed light cnmpared 

 with its components, they give the following law : The sum of the in- 

 tensities of two or more colors is equal to the intensity of the same rays 

 when mixed. (Nature, April, 1886, xxxiii, 525.) 



The committee appointed by the Trinity House to report on the merits 

 of electricity, gas, and mineral oil as light-house illuminants have issued 

 a valuable report giving an account of the investigations carried out 

 under their directions and the conclusions they have arrived at, which 

 are as follows : That the electric light as exliibited in the experimental 

 tower at South Foreland has i)roved to be the most powerful light under 

 all conditions of weather and to have the greatest penetrative power 

 in fog; that for all practical purposes the gas and oil were equal ; and 

 that for the ordinary necessities of light-house illumination mineral oil 

 is the most suitable and economical illuminant. They believe, however, 

 that for salient headlands, important land-falls,and places where a very 

 powerful light IS required, electricity offers the greatest advantages. 

 (Nature, January, 1886, xxxiii,271; November, 1886, xxxv, 41, 60.) 



