some Carbonates of the Calcite Group. 339 



short wave lengths, or it might serve to reduce the import- 

 ance of the long wave lengths, especially for low voltages 

 across the X-ray tube. The dotted curve of fig. 12c shows 

 the relative effect of a screen of tin thick enough to absorb 

 one-tenth of the radiation at A = 0.49 A.U. for a voltage 

 of 60 K.V. The change of the shape of the curve is not 

 very emarkable and the new one does not show enough 

 advantages to repay the loss in useful wave lengths by ab- 

 sorption. Under the ordinary conditions of experimenta- 

 tion if would not be feasible to use higher voltages and cut 

 down the very short wave lengths, less than A = 0.25 A.U., 

 by appropriate screens because of the absorption of these 

 materials in the region of A = 0.45 — 0.49 A.U. 



The material most effective in reducing the intensity of 

 the longer wave lengths is a substance, like aluminum, 

 which shows no selective absorption in the region of the 

 spectrum employed. This kind of filtering would only be 

 serviceable in improving the lower voltage curves. Fig. 

 12d gives the curve obtained by interposing a thickness of 

 aluminum sufficient to reduce A = 0.49 in the 40 K.V. plot 

 to 90 per cent of its unscreened value. Even neglecting 

 the loss of valuable radiation, the result is not as good as 

 the unscreened 50 K.V. curve. Neither does an aluminum 

 filter improve the 50 K.V. curve sufficiently to make its 

 use worth while. 



The conclusions to be drawn from this discussion are : 



(1) That the most useful Laue photographs are ob- 

 tained by operating a tungsten tube, if this is used, at 50 

 K.V. 



(2) That the use of various screens is not desirable. 

 Of course this discussion has assumed that the photo- 

 graphic effect of X-rays was proportional to the amount 

 of their absorption in the silver bromide of the plate, an 

 assumption which seems sufficiently correct for the pres- 

 ent purpose. 



The Calculations from the Laue Photographs. — The 

 symmetry of calcite is that of the point group D 3 d , 25 Of 

 the six space groups which have this symmetry, four have 

 r h , the hexagonal lattice, as the fundamental lattice. The 



n 3 

 calculation of — from the spectrum measurements and the 



nature of the planes producing reflections in the Laue pho- 

 tographs lead to the conclusion that the fundamental lat- 



25 A. Schonflies, Krystallsysteme u. Krystallstructur, 1891. 



