178 0. N. Rood on a Secondary Spectrum. 
The magnifying power of the observing telescope was about 
12 diameters. 
edge of the prism being vertical, the diffraction grating 1s 10 
be revolved in its own plane, somewhat, so that its lines shall no 
2. longer be vertical. This proceeding 
y oF reduces the secondary spectrum te a line, 
‘ and upon viewing it with the naked eye 
or with the observing telescope, it will 
B be found to be curved, and upon slightly 
oo rotating the grating or prism around their 
0 vertical axes, will run through all th 
changes that under the circumstances 
eee: are possible, as is roughly indicated 10 
B vy fig. 2. From this figure it will be seen, 
s oR that we must regard as a true secondary 
8 ™ spectrum a resultant spectrum in whic 
Vv any two, even closely adjacent lines, are 
united; even although the actual unio? 
Application of’ the construction to the secondary spectrum pr? 
uced by two prisms.—Although it has been shown that th 
construction above given applies to the case of a prism pat 
next the collimating telescope when balanced by a diffraction 
grating placed next to the observing telescope, it by no - ee 
immediately follows that, by reversi .g¢ their respective positions, 
results would be obtained which could be accurately predicted 
