204 thp: kadio-activity of matter, 



the entire trajectoiy was marked on the plate. In these photographs 

 the interior of the c^ylinders forming- the screens is strongly atiected 

 ii}' the .secondary emission from the h^ad. The first pictnre (fig. 13) 

 shows that throngh each opening there passes an infinity of raj^s, con- 

 stituting portions of the yjure spectra. These meet with a strip of 

 aluminium 0.1 mm. in thickness, and traverse it without deviation, 

 but not all with equal facility. The slightly deviated rays are pene- 

 trating, and excite secondary radiations when leaving the aluminium. 

 The ver}?^ deviable rays are stopped and give rise to points afl'ected by 

 an intense secondary radiation. 



One only of the two categories of nondeviable rays appears in the 

 I'orm of two fine lines opposite the source; these are very penetrating- 

 rays, the others were arrested quite near the source. 



Another picture (fig. 14) shows the simple beam obtained ])y a double 

 sei'ies of openings: l)v one of them we can sometimes pass two distinct 

 trajectories. 



The third figure is of interest*as it shows the straight beam travers- 

 ing, without deviation, a sheet of aluminium placed obliqu(dy to the line 

 of trajectory; and, finally, the fourth one shows the transmission of 

 simple rays through aluminium, and the secondary effects they produce. 



The same method has enalded me to observe that the secondary rays 

 were themselves deviated by the magnetic field in the same wa}' as 

 the exciting rays. 



The radiations from radium also comprise some which are very pene- 

 trating, consisting- of the least deviable and the nondeviable rays, of 

 which the properties seem to be the same as Rontgen rays. These 

 penetrating rays are but very slightly absorbed, and consequently 

 their action on a photographic plate or on the air is ver}^ feeble, so 

 that by th(^ preceding methods we can get no very exact idea of their 

 intensity. If we interpose in their path a very absorbent screen, they 

 ti averse it partially, l)ut at the same time they ])ecome partially trans- 

 foi-nied into more absoi'bable rays. This transformation recalls that 

 of fluorescence, and, through the sccondai-y action, the effect immedi- 

 atel}' behind the screen is strongei- than if this latter was not there. 

 The photographic plate receiving the radiations — filtered through a 

 thickn(^ss of lead of 1 cm. — gives a strongei' impression through a 

 sheet of lead of 1 nmi. thickness than in the uncovered regions. The 

 diagram (tig. 15) shows the effect of the radiations coming from the 

 sides of a leaden box after having traversed ;"> to 12 mm. of tlu^ metal. 



These secondary i)henomena may partially account for the ap})eai-ance 

 of shadows given by the edges of all the trans))arent screens placed 

 ovei' the photographic ])lates. 



All th(» facts I have just i-elated ha\ c exclusively to do with the 

 obscure i-adiations which traverse opaque bodies, such as metal, glass, 

 mica, etc. But there exists also anothei", (juite different, phenomenon, 

 of which the effects are arr(\ste<l by glass and mica; they are compar- 



