681 



square centimeter is sufficient to produce a density of 1.5 and 0.052 

 mgr. of silver a density of 2.0. 



On the other hand I found in my experiments one fact that at 

 first sight did not agree with m^' deductions. If from the different 

 series of experiments given in (able I and II we calcnlate the in- 

 crease of density with the increase of the action of the light and 

 of the Röntgenrays, we find the fignres given in table III, gi'aphi- 

 cally represented in fig. 7. (See p. 680). 



The first column gives the number of the experiment, the second one 

 the increase of density when the action of light is doubled ; the third 

 one the increase of density on doubling the action of the R(intgenrays. 



In the fourth column tlie (juotient of the figures in the former 

 columns is given. The 5''' column contains the penetrating power 

 of the A'-rays expressed in degrees Benoist. 



The figures in the 4''' column are useful to indicate the amount 

 of difference in slope of the characteristic curves. If these be com- 

 pared with those for (he hardness of the rays, we find (he remark- 

 able fact, that the difi'erence in slope is less for penetrating rays 

 than for rays from a lower-vacuum tube. This is best seen in tlie 

 curve of fig. 7 which shows an absolutely unexpected linear relation 

 between the quotient and the penetrating [lower. The figures in the 

 last column of table HI ai'e calculated with the linear equation: 

 (^uot. : = J .809 — 0.0776 Degrees Benoist. 



Though this seemingly anomalous behaviour of the harder rays 

 might be caused in different ways, we cannot reasonably susi)ect a 

 secondary radiation originated in the gelatine bromide layer itself, 

 as this would occur in every part of the layer. We can only suppose 

 that the more penetrating rays are mixed with an exceedingly 

 absorbable radiation which is present to a far less extent in the 

 radiation of medium hardness. Perhaps a very absorbable radiation 

 might be generated by the harder X-rays by impact on the glass 

 support after their passage through the sensitive layer. If the primary 

 rays already contain a certain amount of soft rays, these may be 

 derived from the anticathode (as a soft characteristic platinum or 

 tungsten-radiation) or from the glass-bulb, or perhaps from the 

 envelope in which the plate was exposed to the rays. Between the 

 first and the last of these possibilities we must have the difference, 

 that in the first case the deepest parts of the gelatine layer contain 

 more silver than the surface, whereas in other cases the surface of 

 the gelatine will be richest in silver. In order to decide in this 

 question I asked Dr. Klessens to make a few microscopic slides 

 from transverse sections of the gelatine layer of different negatives. 



44* 



