Hutchins and Robinson — Concerning Grookes lubes. 463 



All measurements are transferred to other lines, as above 

 described, by lines parallel to the direction between the ends 

 of the scale line and the unit line. The device can also be 

 used in the measurements needed in obtaining edge directions. 



Fig. 2 represents the construction of the axial cross of a 

 monoclinic crystal in which alb: c= 1*092 : 1 .* 0*589 and 

 /3=74° 10'. XX YT ZZ is the isometric axial cross ; OS is the 

 scale line. Oh is sine 74° 10' when radius is OS and trans- 

 ferred to OY is On ; Od is cosine 74° 10' when radius is OS 

 and transferred to OZ is Or ; by completing the parallelogram t 

 and Ot result. For the axial lengths, Oi is 1*092 times OS and 

 Oe is 0*589 times OS and transferred are respectively 

 Oa=l-092xO£ and Oc=0*589xOZ. 



Munich, March 16, 1896. 



Aet. LIII. — Concerning Grookes Tubes ; by C. C. Hutchins 

 and F. C. Robinson. 



We would offer the following contribution to the rapidly 

 increasing literature on the X-rays of Rontgen. It has to do 

 with a part of the subject upon which very little has been 

 written, and for that reason may be helpful to other experi- 

 menters. 



One of the chief difficulties in the way of experimenting 

 has been the cost of the bulbs or tubes. We have proved to 

 our own satisfaction that the making of them need not be 

 beyond the resources of the ordinary laboratory ; for within a 

 few weeks time we have made and tested more than one hun- 

 dred tubes, and have frequently made one and exhausted it and 

 used it, all within an hour's time. All that is required is some 

 little skill in glass-blowing and in the manipulation of the 

 pump. 



The glass. — A hard German glass, or its equivalent, free 

 from lead, has proved the best. It gives a strong green fluo- 

 rescence under the action of the current, and what is of great 

 importance, resists without softening the heat generated by the 

 cathode ray at its point of impact. Unfortunately it is not to 

 be had free from bubbles, and these are the cause of the destruc- 

 tion of many tubes ; the glass being chipped away into the 

 bubble by the action of the current and the tube ruined. It is 

 also rather difficult to put in the electrodes so that they will 

 stay, and it may be necessary to use three kinds of glass, — first 



