INVESTIGATIONS RELATIVE TO ILLUMINATING MATERIALS. 489 



second light-house district in Boston, with such ap]>liances as couhl be 

 procured at the moment, with tlie assistance of Mr. William Goodwin, 

 the acting lighthouse engineer, who took much interest in the subject 

 and rendered efficient service. 



In the erection of a new lamp-shop at the iStaten Island depot, care 

 was taken to make provision for a dark room in which the ])hotometri- 

 cal examinations could be made with more precision than had been ob- 

 tained in the temporary apartments previously used. This room extends 

 the whole length of the building, is about 80 feet long by 12 wide; the 

 windows are closed by iron shutters to exclude the light; and the walls, 

 floor, and all other parts are painted black, after being sanded to remove 

 any glare which might exist. 



In the first experiments on lard-oil the photometrical process employed 

 was that of Rumford, which consists in ascertaining the relative intensity 

 of two lights from their distances from a screen on which shadows of 

 equal darkness are thrown by an intermediate body. In this case the 

 relative intensities sought are indicated by the square of the distances 

 in inches and parts of inches of each light from the screen on which the 

 shadowy are cast. But this method, which is used by the French man- 

 ufacturers of apparatus, and is very simple in theory, does not admit of 

 much accuracy. 



The arrangement, therefore, known as Bunsen's photometer, was in- 

 troduced in its stead, and this, with some peculiar moditications, leaves 

 nothing to be desired. This arrangement consists in jdacing two lights 

 at the extremity of a scale so divided into distances that the relative 

 intensity of the two flames may be immediately read off in terms of 

 candle power when a small intermediate movable screen is equally illu- 

 minated on both sides. This screen is usually formed of a ])iece of white 

 pasteboard of about four inches square, fixed perpendicularly at right 

 angles to the length of the scale, in a sliding frame, by which it can be 

 brought nearer to or farther from one of the lights. In the center of 

 this square is a circular hole of about half an inch in diameter, which 

 is closed by a piece of thin paper, rendered transhujent by a solution of 

 spermaceti in oil of turpentine. This forms a spot which is darker than 

 the other parts of the white screen, and is equally dark on both sur- 

 faces when the screen is receiving an equal quantity of light froui each 

 flame; the screen is moved backward and forward until this eflect is 

 produced, and the index will then point on the graduated scale to the 

 number of the relative power of one of the lights in the terms of the 

 other. 



The screen may also be made of thin paper, the whole of which is 

 rendered translucent except a round spot in the center, of half an inch 

 in diameter. If a light is placed before the s(jreen on one side, the 

 whoh; of the greased part will appear dark, on account of part of the 

 light going through the translucent portion. If, now, another light be 

 placed on the opposite side, an equal portion will be transmitted through 

 the ])eHucid part, and the two surfaces will appear of like intensity when 



