812 HANDBOOK OF PHOTOGRAPHY 



Typical spectrograms taken with the instrument depicted in Fig. o are shown in 

 Fig. 6. The numerical aperture, //12, is sufficient to give short exposures with most 

 ordinary arc and spark sources, and the resolution and dispersion in the ultraviolet are 

 ample for simple spectra, and for absorption spectrophotometry of solutions. 



In purchasing any fixed-focus instrument care should be taken to see that the 

 manufacturer has provided sufficient rigidity to the adjustment of prism and lenses 

 so that they will not readily get out of focus when once adjusted, and that the cassette 

 and plateholder are constructed so thej^ will not warp. Manufacturers should be 

 asked to submit sample spectrograms taken on the instrimient to be purchased and are 

 usually xery willing to do so. 



WTiere higher dispersion is needed than the medium-sized quartz instrument will 

 supply, recourse is usually had to the Littrow type of mounting (see page 810) in order 

 to save space and improve rigidity. The large quartz Littrow is now the most widely 

 used of all spectrographs, being especially suited to spectroscopic analysis of materials. 



Almost any quartz spectrograph can be obtained with glass optical parts which will 

 render it suitable for use in the visible region. While quartz is also transparent in 

 this region, its dispersion is so low as to make quartz spectrographs almost valueless 



Fig. 5. — Baiisch and Lomb medium quartz spectrograph. 



at wavelengths longer than about 5200 A., beyond which, also, the ordinary photo- 

 graphic plate becomes insensitive and special plates must be used. 



The dispersive properties of even heavj^ flint glass are not so great as is desirable, 

 and several prisms are sometimes used in train to increase dispersion and resolving 

 power in the visible region. In general the concave diffraction grating, with its uni- 

 form dispersion, usefidness for all wavelengths, and simplicitj^ of adjustment, is to be 

 preferred to prism spectrographs when high resolution and dispersion are important. 



While grating spectrographs can be obtained from certain manufacturers, satis- 

 factory gratings are so difficult to obtain that the grating spectrograph is not standard. 

 The great usefulness of the grating in the past has come largely from its high dispersion 

 and resolving power, and since gratings of 6-in. aperture or over are not unusual, 

 while prisms of over 2-in. aperture are rare, grating spectrographs ordinarily have 

 three or more times the focal length of prism instruments. On account of their size, 

 large grating instruments ordinarily are assembled bj- their user and operated uncov- 

 ered in a darkened room, though standard commercial models in the smaller sizes are 

 appearing on the market. 



Table I. — Mountings and Their Characteristics 



Rowland Spectrum of uniform dispersion; simple to change region 



Abney Same as Rowland, but source and slit move while grating and camera are fixed 



Eagle Long and narrow, relatively low astigmatism, reaches higher orders 



Paschen Broad spectral coverage with single exposure 



Wadsworth Stigmatic, and of high light-gathering power; best for most purposes of applied 



spectroscopy 



