USE OF SPECTROSCOPE IN ASTRONOMICAL OBSERVATION. 147 
the magnifying power of this telescope. Over one half of the 
slit d is placed a right-angled prism e, for the purpose of reflect- 
ing the light received from the mirror /, through the slit, and 
thus comparing the spectrum of the star or other celestial object 
under examination with that of light from any other source. 
Behind the slit is placed a collimating lens at a distance equal 
to its focal length. The diameter of the lens is such that it 
receives the whole of the light diverging from the linear image 
when the latter is brought exactly within the jaws of the slit. 
The dispersing portion of the spectroscope consists of the two 
prisms h, h. The refracting angle of these prisms will be dif- 
ferent according to the purposes which the instrument is meant 
to subserve. In his researches on stars Mr. Huggins employed 
a spectroscope having two prisms (made by T. Boss), each of 
60 degrees ; but for many of his most important researches he 
' found it advantageous to employ a spectrum-apparatus con- 
structed by Mr. Browning, in which one of the prisms had an angle 
of 35 degrees, the other an angle of 45 degrees. The spectrum 
I is viewed through a small telescope I, having an adjustment for 
I level at m. At the focus of the object-glass of this telescope 
are fixed two wires at right angles to each other. These are 
viewed together with the spectrum by the positive eye-piece p. 
The telescope is carried by a micrometer-screw g, the centre of 
motion being so situated that the rays forming any part of the 
I spectrum pass centrically through the object-glass when the 
I telescope is so placed as to receive such rays. 
I It will be seen from this description that there are two ways in 
I which the position of any line in the spectrum of a star can be 
determined. Either it may be compared with the bright lines 
, seen in the spectra of different metallic elements, these spectra 
being brought into comparison with the star’s spectrum by 
means of the mirror/; or else, when any line in the star’s 
, spectrum has been identified with some solar line, we may first 
bring that line into coincidence with the cross-wire in the focus 
of the object-glass, and note the reading of the micrometer- 
screw ; then move the telescope until the line whose position we 
require to determine is brought to the cross-wire, and again 
note the reading of the micrometer-screw. The difference 
between this and the former reading indicates the distance 
between the two lines. 
Of course other modes of measuring the arc through which 
the telescope has moved are available. Into these we need not 
! enter, as every observer will be familiar with the contrivances 
suitable for such a purpose. 
It must be noted that a spectrum mapped out in this way 
will not agree with the spectrum of the same object similarly 
mapped out by means of another instrument, because different 
