810 HANDBOOK OF PHOTOGRAPHY 



through tlie prism, and retraverses the collimator, which behaves now as a camera 

 lens and l)rings the spectrum to a focus on P. 



Two defects keep the Littrow mounting from displacing all other types of prism 

 mounting to the extent that its simplicity and compactness would lead one to expect. 

 The proximity of slit and plateholder requires the introduction of a reflecting prism 

 or other device to separate the incoming and outgoing beams, and the reflection and 

 scattering of light from the front face of the collimator directly back to the photo- 

 graphic plate may cause objectionable fogging which is hard to eliminate. This can 

 often be decreased either by tipping the lens slightly, which will introduce a certain 

 amount of astigmatism but throws the reflected light above the plate, or by intro- 

 ducing stops and diaphragms at strategic points. In any event the inside of the 

 case surrounding a Littrow mount should be thoroughly blackened and numerous 

 baffles should be used to cut down stray light. 



To locate the cause of plate fogging, when this is found to occur with a Littrow 

 mounting, the slit may be widened to its fullest extent with a high-intensity incan- 

 descent lamp placed in front of it. On looking at the collimator lens through the 

 horizontal slot which admits light to the cassette, the location of any bright points of 

 light should be noted, and these should be eliminated by some means which will cut 

 off as little as possible of the main beams of light. In order to carry out this operation 

 successfully, it is sometimes necessary to eliminate as much as one-fourth of the lens 

 aperture with diaphragms. In attacking straj^ light a polished reflecting surface 

 which directs an unwanted beam of light into a dark pocket is often more satisfactory 

 than a rough blackened surface which may scatter some of the radiation falling on it. 



In one widely used and excellent type of Littrow instrument having quartz or glass 

 optical parts, the length of the case is over 7 ft., but as the optical sj^stem may be 

 considered as having been folded together in the middle by use of the autocollimation 

 principle, the dispersion obtained is equivalent to that furnished by an instrument 

 twice as long. 



The prism and lens are mounted on a carriage which moves along a slide, their 

 positions on this being determined by means of a scale and index. The prism can be 

 rotated to throw various regions of the spectrum on a 4- by 10-in. plate, which is held 

 in a cassette-plateholder combination which in turn can be rotated to bring it into 

 coincidence with the focal curve for any spectral region between 2000 and 8000 A. 



Plane-reflection gratings are almost always mounted in the Littrow manner. A 

 grating so arranged has the advantage over a concave grating in the Eagle mounting 

 (page 813) of giving stigmatic images, with a resulting increase in brightness and 

 resolution in the higher orders. A lens carefully corrected for chromatic aberration 

 must be used in all orders except the first, however, for otherwise the various over- 

 lapping orders will not be brought to a focus on the same cvirve. 



Stigmatic and Astigmatic Spectrographs. — A camera lens or mirror, unless especially 

 designed, produces a true image of an object only when image and object are close to 

 the optic axis. As the beam angle departs more and more from the optic axis, greater 

 amounts of astigmatism are introduced, the rays being brought to one line focus at a 

 certain distance and to a second line focus perpendicular to the first at a greater dis- 

 tance. In common types of prism spectrographs the astigmatism can usually be 

 neglected, as extremely fine focus is needed only in the horizontal direction to resolve 

 close spectrum lines and a focus only one-tenth as sharp will serve in the vertical 

 direction. 



The spectrum lines produced by a concave grating as ordinarily used are astigmatic 

 images of the slit, each illuminated point of this slit being imaged as a. vertical line in 

 the Rowland circle. No decrease in the purity of the spectrum results so long as the 

 slit is accurately parallel to the rulings of the grating and if neither the slit nor the 

 astigmatic images are curved. With most gratings a slight line curvature does exist, 



