192 



IOWA ACADEMY OF SCIENCE 



In experimenting with crystals which absorbed X-rays only 

 slightly, it was found that the lines on the photographic plate 

 became wide and overlapped, making it impossible to perceive 

 any spectral lines whatever, the plate giving the apearance of 

 a continuous spectrum. A study of the conditions which would 

 cause this led to the conclusion that the resolving power of a 

 crystal, or its ability to separate lines of nearly the same wave 

 length could be increased by making the crystal very thin. A 

 test was made, using a crystal of rock salt which had been 

 ground to a thickness of 0.2 mm. The results amply justified 

 tlie theory as the lines were narrower and sharper and lines 

 appeared in a region which had previously been considered as 

 a region of continuous spectra only. A print from this plate 

 is shown in figure 18. 



It is impossible to reproduce the finest lines which appear on 

 the plate. In taking this photograph the crystal was first ro- 

 tated through the position which would give the tungsten spec- 

 trum on one side of the center line and then reversed to give 

 the spectrum on the other side. In this way the distance be- 

 tween the two positions of a line is twice the displacement dis- 

 tance of that line. In this case the crystal was rotated about 

 ail axis passing through one of its faces and which was 15.553 

 centimeters from the plate. The distance between the two po- 

 sitions of the strong line of greatest wave length was 18.248 

 centimeters. One-half of this, 9.124, when divided by 15.553, 

 gives the tangent of twice the angle of reflection which is found 

 to be 30° 24'. The angle of reflection is therefore 15° 12'. 



In the following table the values of the X-ray wave lengths 

 cf tungsten are given. The computations are based on the value 

 of 2.814xl0- s cm. as given by Bragg for the distance between 

 planes in a rock salt crystal. 



Fig. 18. 



