24 J. C Draper—Projection of the Fraunhofer Lines, ete. 
in the first, and by the apparatus described above a spectrum 
of a length of more than thirty centimeters is obtained. 
For the projection of the prismatic spectrum a prism is sub- 
stituted in place of the grating, when a very fine spectrum is 
produced, the focus of the violet end of which is very much 
closer to the prism than that of the red end. 
In the diffraction spectra, also, it is necessary to vary the an- 
gle at which the screen is placed to define sharply the lines at 
the extremities of each spectrum. In the spectra of the first 
order on each side, the screen is placed very nearly at right an- 
gles toa line drawn from the grating to (6), in the spectrum. 
As each order in succession is examined, the divergence from 
this angle is greater and greater, and at the same time the focal 
distance of the lines moves nearer to the grating. 
The lenses I have employed were those of a very fine photo- 
graphic combination; they give with the rest of the arrange- 
moe wider, when each spectrum will show the characteristic 
absorbent bands of the substance employed, the position being 
indicated (and if required, recorded) by their relation to the 
ines of the solar spectrum in which they are produced. 
When the calcium or electric light is to be used for lecture 
toom demonstration of diffraction spectra, the lens (B) should 
have as short a focus and as large a diameter as possible. The 
grating may also be so arranged on its vertical axis as to throw 
its image at a right angle to the line B, H, to be there received 
onascreen. ‘Though by this device the spectra on one side of 
the image of the grating are greatly elongated and those on the 
other compressed, it presents the advantage of enabling the 
audience to see all the spectra at once, and also the optical con- 
trivances by which they are produced. 
_ November 25, 1874. 
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