SPECTROSCOPIC PHOTOGRAPHY 



815 



internal limiting aperture remain constant in area and filled with light of each 

 wavelength considered, nothing done outside the spectrograph will increase the 

 illumination which passes through it from a given source. With certain types of 

 source it may be difficult to fulfill these conditions, in which case auxiliary focus- 

 ing apparatus is desirable. But the method, frequently suggested by the beginner, 

 of using a collecting lens of very short focus and high aperture, produces no increase 

 in intensity. 



Certain manufacturers provide optical benches attached to their spectrographs to 

 aid in alignment of light sources. In lining up an optical bench or a source-lens com- 

 bination, the lens should first be removed and the light from the unaided source be 

 allowed to fall on the widened spectroscope slit. The cover of the spectroscope, if this 

 be enclosed, should then be removed and a white card held directly behind the colli- 

 mator lens. The source may be shifted until the small spot of light coming through 

 the slit is centered in the collimator both horizontally and vertically. The collector 

 lens should then be placed in position to focus the source on the slit, if this type of 

 illumination is desired, and the collimator should now be found filled with light when 

 the slit is narrowed to any desired extent. If the light beam falling on the collimator 

 more than fills this, the collecting lens should be stopped down or a lens of different 



Fig. 9. — Illumination of spectrograph. 



focal length should be used, since overfilling the collimator may give rise to undesirable 

 scattered light inside the spectrograph. 



The collecting lens should, needless to say, be transparent to the entire spectrum 

 region transmitted by the spectrograph with which it is to be used, but need not be of 

 high quality. Ordinary spherical lenses of spectacle quality will usually serve, though 

 when the maximum intensity at all wavelengths is needed it is desirable to use a lens 

 free from chromatic aberration. In using uncorrected quartz lenses, it should be 

 kept in mind that the focal length for the ultraviolet rays is much shorter than that for 

 the visible, so focusing should be done with a fluorescent screen or by deliberately 

 bringing the source closer to the lens so as to throw the visible beam somewhat out of 

 focus. 



A concave miri'or can be used instead of a lens for focusing light on the slit, though 

 usualljr with some loss in intensity and with difficulties due to astigmatism substituted 

 for those arising from chromatic aberration. * Glass mirrors covered with a thin coating 

 of aluminum have high reflecting power throughout the visible and ultraviolet regions. 



Adjustment and Focusing of Spectrographs. — Commercial spectrographs of fixed 

 focus are usually shipped with their optical parts in adjustment, and instruments with 

 variable setting are provided either with calibrated controls or a table of scale settings. 

 Occasionally mechanical or optical parts are moved in shipment and readjustment may 

 be necessary. 



After unpacking, the cover of the instrument should be i-emoved and the optical 

 surfaces of prisms and lenses wiped clean with a soft piece of cloth, or with Japanese 

 rice paper or the leiis paper sold by optical manufacturers. The faces of diffraction 



