112 Dr. J. W. Draper on the Distribution of 



After the intromitted sunbeam has passed through the prism for 

 a short time, the temperature of that mass has risen ; and the heat 

 from this source has become intermingled with the proper spec- 

 trum heat. But this error is very easily eliminated. It is only 

 necessary to put a screen n in the path of the incoming ray be- 

 tween the slit and the prism and note the deflection of the mul- 

 tiplier. Used as we are here supposing, the multiplier has two 

 zeros. The first, which may be termed the magnetic, is the po- 

 sition in which the needles will stand when no current is passing 

 through the coil. The scale of the instrument should be set to 

 this. The other, which may be termed the working zero, is 

 found by coupling the pile and the multiplier together and in- 

 troducing the screen n between the intromitting slit and the 

 prism. On doing this it will probably be found tliat the index will 

 deviate a few divisions. Its position should be accurately marked 

 at the beginning and close of each set of measurements, and 

 the proper correction for them made. The disturbing influences 

 of the mass of the prism, of the mirror, and of the pile itself are 

 thus eliminated. As respects the last, it should not be forgotten 

 that it may be affected by changes in the position of the person 

 of the experimenter himself. 



With the intention of diminishing these errors I have usually 

 covered the upper and lower portions of the concave mirror dd 

 with pieces of black paper, so arranged as to leave a band across 

 the middle of sufficient width to receive and reflect the entire 

 spectrum. In fig. 4 « « is the upper paper, b b the lower, c c the 

 uncovered reflecting band receiving the spectrum r v. Had the 

 spaces thus covered been permitted to reflect, they would have 

 rendered more intense the image of the prism and its extraneous 

 heat. 



As regards the multiplier, care must be taken to avoid dis- 

 turbance from aerial currents. I have one of these instruments 

 of French construction v/hich could not be used in these delicate 

 researches until proper arrangements were applied. It was co- 

 vered with a glass shade. The slightest cause occasioned cur- 

 rents in its included air, which perpetually drifted and disturbed 

 the needles. For this reason, and also for more accurate read- 

 ing, it is best to view the position of the index through a small 

 telescope. 



The combination of needles being nearly astatic, attention 

 must be paid to their magnetic perturbations, whether arising 

 from local or other causes; and since the vibrations are very 

 slow, ample time must be given before the reading is ascertained. 



The condition of the face of the pile is of importance. It 

 must be such as to extinguish as completely as possible all the 

 incident rays. To paint it with lampblack mixed with gum will 



