2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 87 



length as possible. In other words, one wishes to work with a wide 

 and long slit and as large monochromatic images of the slit as is com- 

 patible with essential spectral purity. A second demand is that pro- 

 vision be made for a nonselective determination of the relative intensi- 

 ties of the different wave lengths incident upon the organisms. A third 

 demand is that provision be made for the exposure of the organisms 

 without great hazard of contamination. 



These three requirements determine the general character of the 

 combined recording monochromator and biological spectrograph which 

 has been constructed. Since one of the most interesting regions from 

 a biological standpoint lies between 2,500 A. and 3,000 A., fused 

 quartz serves very well as the material for the construction of the 

 optical parts. The difficulty which such material presents due to its 

 slight inhomogeneity is not of great moment in this connection. The 

 resulting loss of definition is of no consequence because of the wide 

 slits demanded by the coarse-structured biological plates. In order 

 that a comparatively high degree of spectral purity might be maintained 

 despite the use of large slit-widths, a relatively great dispersion is 

 required. This is obtained by the use of two 60° prisms, together with 

 focal lengths of collimator and telescope of 50-60 cm. In order to 

 maintain a high light intensity, unusually large prisms have been em- 

 ployed, yielding a numerical aperture of f . 4 to f . 5 depending on the 

 wave length. The second prism is slightly larger than the first, the 

 first being 13 cm high x 13 cm diagonal face, the second 14 cm high x 

 16.5 cm diagonal face. The use of a larger second prism minimizes 

 the reduction in aperture for the beams of least and greatest devia- 

 tion. The optical arrangement is shown diagrammatically in Figure i. 

 With the high numerical aperture, it was necessary to have the lenses 

 ground aspherically in order to minimize spherical aberration. Since 

 it was desirable that the instrument should be used efficientlv over 

 a wide range of wave lengths either as a spectrograph or mono- 

 chromator, provision was made to swing both collimator and camera. 

 The first slit " s " is curved, compensating for the change of prismatic 

 deviation ofif the axis and thus yielding straight line images of the 

 slit. A slit-length as great as 5 cm proves to give satisfactory images. 

 Provision is made for the independent focusing of both collimator 

 and camera. The lenses are 16.5 cm in diameter. The camera has 

 been specially constructed to serve first, with a conventional plate 

 holder, for photographic purposes, and second, with a special plate 

 holder which can be sterilized in an autoclave, for biological purposes. 

 This plate holder is constructed of metal and provided with both 

 shutter and quartz window so as to isolate the biological material 



