1902.1 on X-Hays and Localisation. 129 



while a sharp shadow of an object is being cast on a photographic 

 plate by the point on the anode, a blurred image is being produced 

 from the large and diffuse area of the bulb. These rays from the 

 glass may be called secondary, and photographically are about six to 

 ten times weaker than the primary rays. 



A strip of lead J inch wide and too thick to let any X-rays pass 

 through, was applied closely to the tube — it cast a shadow — no 

 primary X-rays could reach the plate within the shadow ; but the 

 two halves of the fluorescing glass supplied diffuse X-rays, and the 

 slide shows two shadows of a single wire interposed. 



To cut off the superfluous rays which fog and blur the true 

 X-ray image I surround the tube with non-conducting opaque ma- 

 terial — red lead, red lead and plaster, or place it in a box lined 

 with a mixture of white and red lead with a small hole in the lid 

 to act as a diaphragm. 



The properties of X-rays may be very shortly summed up as 

 follows : — they proceed in straight lines from their point of origin 

 on the anode to the destination, be it a photographic plate or a 

 fluorescent screen. They are invisible. They cannot be refracted, 

 and they cannot be reflected, except to a small extent, and then only 

 a scattered reflection, just as light is reflected from white paper. 

 They cause certain substances to fluoresce. (This screen was kindly 

 made for me by Messrs. Johnson and Matthey.) They darken a 

 photographic plate ; they pass through substances in the order of 

 their atomic weights, the lighter and less dense the substance, the 

 more readily do they pass through, and therefore all organic sub- 

 stances are comparatively transparent, while metals are more or less 

 opaque according to the thickness ; they also have the power of dis- 

 charging electrified bodies. The tube when worked by a coil with a 

 rapid interrupter is illuminated continuously, and the eye gets the 

 impression that the emission of X-rays is continuous, just as the 

 emission of light from a candle, but the impulse of the coil is inter- 

 mittent, the impact of the cathodal ray is intermittent, and the out- 

 put of the X-rays is intermittent. If we reduce the speed of the 

 break the light in the tube is seen to flicker, if the speed of the 

 break is increased the light apparently becomes continuous. This 

 is due to a limitation in the human eye. If an impression of light 

 is given to the eye, however transient, it lasts for one-tenth of a 

 second, and if the impressions are repeated oftener than ten times 

 a second, the impression upon the eye is that of a continuous illu- 

 mination. The intermittence of the highest rate of speed is shown 

 by the rapid rotation of a small fluorescent screen. This experiment 

 shows that the light produced in the fluorescent screen is also 

 transient. This luminescence is in marked contrast to the lasting 

 luminescence which is termed phosphorescence. 



During Lord Rayleigh's recent lectures here on Saturday after- 

 noons on recent Electric Developments, I was much interested in his 

 method of producing a very sudden break in the primary of the coil 



Vol. XVII. (No. 96.) k 



