180 ANNUAL OF SCIENTIFIC DISCOVERY. 



M. Helmoltz considers only the first and third positions, reserving 

 for a special memoir a discussion of the views of Brewster. He en- 

 deavors to determine the colors produced by a mixture of two or 

 more colors of the spectrum, making use of the following arrangement: 



The source of light was a slit made in the form of a V, cut in a 

 black screen, and illuminated by the light of the clouds, or by a large 

 white screen, upon which the sun's rays fell ; the two branches of the 

 slit were, inclined 45 to the vertical and were consequently, at right 

 angles with each other ; at the distance of 14 feet was placed a prism 

 of llint glass, its refracting angle vertical, in the position of minimum 

 deviation, -the rays refracted by this prism were immediately re- 

 ceived upon the object-glass of a small telescope which served the 

 purpose of observation. Upon covering one of the branches of the 

 slit with an opake screen,- one could perceive by the aid of the teles- 

 cope, a spectrum of the form of a parallelogram, having two sides hor- 

 izontal, and two parallel to the slit through which the light passed. In 

 this spectrum the principal rays of Fraunhofer were distinctly visible, 

 arranged parallel to the slit, consequently, upon permitting the light 

 from both slits to fall upon the telescope at the same time, two spec- 

 trums were formed marked by bands parallel to the two branches of 

 the slit, and which partly covered each other ; the slit was of such 

 dimensions that one colored band of one spectrum, crossed all the 

 colored bands of the other, which thus permitted one to examine the 

 effect of the combination throughout ; in looking through the telescope 

 all these combinations Avere simultaneously visible, and to judge cor- 

 rectly of the resulting tints, it was necessary to examine them sepa- 

 rately. To effect this, the telescope was directed so that the intersec- 

 tion of the cross hairs in the focus of the eye-piece, was upon the 

 combination it was desired to examine, and then the eye was with- 

 drawn 25 or 30 inches from the eye- piece in the direction of the axis 

 of the telescope, in this position only a very small space in the centre 

 of the field could be perceived, enabling one to judge of the color 

 without being influenced by the neighboring colors. This position 

 of the eye was maintained by means of a hole pierced in a large 

 black screen. When the compound tints had been thoroughly deter- 

 mined the two composing tints were observed by covering succes- 

 sively one and the other" branches of the slit. Whenever the tint 

 approached the white, it was estimated by comparison with a large 

 sheet of paper, perfectly white, which encircled the eye-piece of the 

 telescope. 



In short, to vary in a known ratio, the relative intensity of the two 

 component tints it sufficed to change the prism from the vertical posi- 

 tion and incline it as might be necessary ; by this arrangement the 

 form of each spectrum was changed, one approaching the form of a 

 rectangle, and the other elongating more and more, the surfaces of the 

 two spectrums, and consequently the different colored bands ceased to 

 be equal, and since each spectrum Avas produced by the same quantity 

 of white light, the intensities at the corresponding points were changed, 

 and since it was easy to determine the ratio of the surfaces of the two 



