1896.] 



21 



Crookes tube is not obtained by direct connection to an ordinary 

 Rhumkorfif induction coil, even of a large size. So called 'high fre- 

 quency currents,' however, appear to give good results, and our own 

 experiments have been made with a tube excited by current, obtained 

 from the secondary circuit of a Tesla oil coil through the primary of 

 which were continually discharged twelve half gallon Leyden jars, 

 charged with an alternating current of about twenty thousand volts 

 pressure, produced by a transformer with a spark gap across its high- 

 pressure terminals." 



Having no such apparatus as this at the University, and thinking that 

 possibly some indication might be obtained from a simpler arrange- 

 ment, w^e left out the second coil and joined the tube directlj' to the 

 secondary of the tirst coil. The coil we are using was constructed by 

 Apps, of London. It has a primary resistance of about 0.3 of an ohm, 

 and a secondary resistance of about 3200 ohms. The Crookes tube 

 which is one of the collection in the physical cabinet at the University, 

 is a shadow tube nearly twenty-flve centimeters long and eleven centi- 

 meters in diameter at its larger end. 



The first result that was unmistakably a success was obtained on 

 "Wednesday, February 5. A small slip of glass and a piece of sheet 

 lead, together with a wedge of wood, were held in place upon a sensi- 

 tive photographic plate by elastic bands, and the whole enclosed light 

 tight in an ordinary plate holder. This was placed horizontally upon a 

 table, eight or ten centimeters below the large end of the Crookes tube. 

 An exposure of twenty minutes produced, upon development, a sharp 

 impression of the objects, the glass and lead appearing opaque, while 

 the portion of the plate covered by the wood was hardly less affected 

 than the parts entirely unprotected. The sight was startling at first as 

 every experimenter who gets the result for the first time can testify. 

 This experiment was immediately followed by an attempt to obtain a 

 skeleton view of the hand, the result of which will be shown by the first 

 slide. 



From that time until the present, many experiments of a varied 

 nature have been tried, the object being to investigate substances with 

 reference to their transparency ; to detect, if possible, refraction or re- 

 flection ; to determine the action of various crystals cut in different 

 ways with reference to the optic axis ; and a few experiments to test 

 the possible efficiency of a special method of treating the sensitive film. 



Early associated with the w^-iter was Dr. H. W. Cattell, who obtained 

 some very curious cases of malformation of the hand and fingers, and 

 produced results which have proved extremely interesting from a surgi- 

 cal point of view. 



Our experiments on crystals have not resulted in much that is inter- 

 esting, except, perhaps, in one case which I will refer to presently. 

 One plate exposed had upon it a tourmaline, a double image prism, a 

 Nicol prism, an amethyst, an irregular quartz crystal, some mica discs. 



