204 RADIATION BIOLOGY 



In the visible and adjacent regions the refractive index for most mate- 

 rials increases with decreasing wave length, and therefore the shortest 

 wave lengths are most deviated. A plot of the refractive index against 

 wave length shows that the slope of the curve dyi/dX decreases with 

 increasing wave length. As a result, the dispersion dB/d\ is not uniform, 

 and the longer wave lengths usually are relatively crowded as compared 

 with the shorter. For common prism materials the cHspersion may be 

 ten times as great at one end of the useful range as at the other. This 

 lack of uniform dispersion is one of the principal limitations of the prism 

 spectroscope. 



Liquid prisms have been the subject of much interest for large-aperture 

 irradiation monochromators, since large dimensions can be obtained 

 inexpensively. However, they have not been extensively used because 

 of inhomogeneities caused by convection currents and the high temper- 

 ature coefficients of refraction (Sawyer, 1944) as compared with those of 

 solids. Harrison (1934) has described a reflecting liquid-prism mono- 

 chromator consisting of a concave mirror immersed at an angle in a tray 

 of water. This arrangement eliminates vertical windows and offers some 

 possibilities for obtaining high irradiances. 



2. Diffraction grating. The diffraction grating consists of very fine 

 and closely spaced lines, ruled on an optical surface, which behave as 

 shts in transmission gratings and fine mirrors in reflection gratings. Dif- 

 fraction of the radiant energy by the ruhngs produces a series of over- 

 lapping spectra by interference. Ideally the lines are shallow V-shaped 

 grooves formed by a ruling engine drawing a diamond point over the 

 surface (Fig. 3-225). Early transmission gratings were ruled on hard 

 surfaces such as glass and metal, but these materials rapidly eroded the 

 ruhng diamond. The development of the vacuum-evaporated aluminum 

 mirror, with its soft surface and high reflection efficiency throughout the 

 spectrum, made possible reflection gratings of large size and precisely 

 controlled groove contour. 



Standard rulings for the visible and ultraviolet are nominally 7500, 

 15,000, and 30,000 lines per inch (300, 600, and 1200 lines per milhmeter) ; 

 coarser ruhngs are used in the infrared. Plane gratings are ruled on opti- 

 cally flat aluminized mirrors and are available commercially with a ruled 

 area of 10-20 cm square, and much larger plane gratings are in prospect. 

 The concave grating is made by ruhng on a long-focal-length aluminized 

 parabohc mirror. Because of factors concerned with astigmatism, the 

 lines are usually short, not over 5 cm long, although the ruled area may 

 be much longer. Consequently the concave grating has a smaller ruled 

 area than a plane grating with the same over-all dimensions. 



Because of the great cost of producing original gratings, various 

 methods of transferring the ruling contour of original gratings to other 

 optical surfaces have been developed. These duplicates are known as 



