August 1, 1889.] 



KNOWLEDGE ♦ 



199 



abrasions. The whole of this aide (presumably the side on 

 which he was leaning against the tree) had exactly the 

 appearance of an exaggerated example of post-mortem stain- 

 ing ; the other sawyer had his legs burnt in places ft'om the 

 point where he had been lesting his knife downwaids. The 

 legs were cut probably by the steel buckles he had worn. 

 . . . Both men were so scorched that no lines were visible 

 to indicate the course taken by the electric current, although 

 these subsequently came out as shown in the illustration." 



It is evident from tliLs description that the skin of the 

 men must have been subjected to great heat as well as to 

 pressure, which caused the abrasions and the bruising. 

 The' fact that the injury done was chiefly to the skin and 

 surface tissues shows that the seat of the explosion was on 

 the surface of the body. The current seems to have 

 ti'avelled along the damp surface of the skin and to have 

 driven the moisture into vapour, which blew the clothes 

 outward in rags without singeing them. 



It is a curious fact that the boots of lightning-struck 

 persons are nearly always burst open. The sawyers' boots, 

 with torn " uppers," may be seen on the top of the small 

 barrel in Mr. Marriott's photograph, and the children's 

 boots show evidence of a disruptive force upwards, not in 

 the direct line of discharge to or from the earth and the 

 child. This is what might be expected on the theory that 

 the explosion is due to moisture on the skin, which is driven 

 into vapour. The leather of the boot wovild confine the 

 perspiration, and the weakest part of the boot would go. 

 We shall probably learn more as to the action of the electric 

 current in producing death when the American electric 

 methods of execution have been in use for some little 

 time. From the facts before us at present it would seem as 

 if a dry-skinned criminal would suffer differently from one 

 who had struggled on his way to the scaffold and was 

 perspiring fieely. 



The very interesting photograph of lightning in the 

 lower left-hand corner of the plate was taken by the Rev. 

 A. Rose, at Emmanuel College, Cambridge, on the evening 

 of June G last. It shows the main flash biight, with dark 

 branches spreading away from it on either side. In all other 

 photographs of branching flashes the main flash has been 

 brighter than the bi-anches ; we may, therefore, probably 

 assume that in this instance the main flash was the 

 brightest, and that the fainter flashes are reversed, while 

 the brighter flash is not reversed. On lookmg more closely 

 into the photogi-aph, one sees that the main flash is 

 boi'dered along its edges by a dark fringe corresponding to 

 the hazy edge of the image of a bright object, described in the 

 last number of Knowledge as due to the optical imperfec- 

 tions of the instrument with which the photograph was tjiken. 

 This dark border to the main flash can just be seen on the 

 photographic copy published with this. It will also be 

 noticed that the brighter parts of some of the branches, 

 notably the middle one on the left-hand side, are only 

 partly reversed. In the glass transparency which I have 

 before me, one can see that a narrow line of light runs 

 down the central part of the branch near to whei-e it joins 

 the main flash. Evidently the brighter parts of the flash 

 have given rise to bright images, and the less bright parts 

 have reversed the photograph ic action produced by the 

 background of sky, and have given rise to an area on the 

 photographic plate which is darker than the surrounding 

 area of sky. Taken in conjunction with the experiments 

 described by Mr. Clayden in his letters, and with the older 

 experiments on the reversing action of the less refrangible 

 rays, referred to in the letter of Sir G. G. Stokes, we seem 

 to have material here for a very interesting research. The 

 background of sky was lit up by summer twilight 

 rich in long wave-lengths, the blue end of the spec- 



trum having been sieved out by the long passage 

 of the solar light through the lower air. The light 

 of the lightning, richer in short wave-lengths, seems 

 to have first bleached or reversed the photographic 

 action of the r'cddish Ught of the sky, and then to have pro- 

 duced its own positive effect. K this theory is correct, 

 how is it that this particular photograph of a brandling 

 flash is the only one which shows the branches and fainter 

 parts of the flash reversed i Possibly this photograph was 

 taken on a background of clear sky, while other flashes 

 have been photographed on a background of cloud. We 

 need further observations. In experimenting, one is 

 generally on the verge of discovery when one comes to 

 something one does not understand. Here there is a great 

 deal which needs explanation. If the reversing effect of 

 one region of the spectrum with respect to another is proved, 

 more rapid photographic action may possibly be obtained 

 by eliminating the reversing wave-lengths by passing the 

 light which is to form the image to be photographed 

 through a coloured medium, which will sieve out the 

 reversing ray.s. 



THE COMMON COCKROACH.-I. 



By E. A. Butler. 



NDER this name some may perhaps hardly 

 recognise the insect so well known as a 

 kitchen nuisance, and popularly called a 

 '' black beetle." A more inappropriate 

 name than black beetle could hardly be 

 conceived : the epithet " black " is appa- 

 rently applied in a loose sort of way to 

 indicate merely a dark colour-, for, when closely examined, 

 the creature is seen to be really i eddish brown of a deeper 

 or brighter tinge according to age and sex, only approaching 

 black in the older females, and then merely on the back. 

 Again, in many important structural characteristics as well 

 as in the nature of the 

 changes it undergoes in 

 the course of its life, it 

 is widely removed from 

 the true beetles. Not but 

 that there tn-e black bee- 

 tles {Blaps, &c.) rightly 

 named, that domicile 

 themselves with man, 

 lurking in cellars and 

 outhouses ; but these are 

 totally different insects 

 from Periphineta orien- 

 falis (fig. 1), the common 

 cockroach, with which 

 we ai-e now concerned, 

 and they never appear 

 in enormous swarms in 

 our kitchens as the cock- 

 I'Oach frequently does. 



EngUshsoildid not pro- 

 duce this much -maligned 

 insect ; it is an immi- 

 grant from foreign parts. It is, in fsvct, not an inhabitant 

 of temperate climates at all, but came originally from 

 the tropical p.arts of Asia. While importing cargoes of 

 the productions of other countries, we often unwittingly and 

 unintentionallj' add considerably to our own insular fauna. 

 Probably no shipload of animal or vegetable pix)duce from 

 distant lands starts for our ports without the accompaniment 

 of an assemblage of living ci-eatures, chiefly insects, from the 



Fig. 1.— The Common Cockuoach 

 (Pfripla)ieta ori^ntalis) Female, 



