July 30, 1 8 74 J 



NATURE 



245 



Photographic Irradiation 



As the question of whether irradiation is due to the imperfec- 

 tion of the instruments, or to an action taliing place witliin the 

 thiclvness of the collodion film, is a matter of considerable im- 

 portance in all cases in which photography is made use of for 

 the purposes of accurate measurement, I have repeated and some- 

 what varied the experiments which have lately been described in 

 Nature, vol. x. pp. 205, 223, by Mr. Ranyard. I therefore laid on 

 a uranium dry plate a piece of platinum foil, and with full aperture 

 of lens took, with an exposure of twenty-five minutes, a photo- 

 graph of a piece of cardboard, in which were four parallel slits, 

 hung against a background of bright sky. In spite of the long 

 exposure, the images of the slits are sharply cut off at the place occu- 

 pied by the edge of the platinum foil, though at the same time 

 there are very marked traces of the outer hazy irradiation arising 

 from reflection from the back of the plate. I then took with 

 the same exposure, and under' what seemed to be similar con- 

 ditions of illumination, a photograph of the same cardboard 

 sheet, on an extra-sensitive Liverpool plate, and again found that 

 the images of the slits were sharply cut off. This seems to me 

 to decisively show that the irradiation cannot be due to a spread- 

 ing within the film, caused by the light dispersed from the 

 highly illuminated particles in the collodion, as suggested by 

 Mr. Aitken ; and I feel inclined to agree with Lord Lindsay and 

 Mr. Ranyard that it must be due to some cause that has its seat 

 of action in front of the collodion film. 



Bedford W. C. Crofts 



Feathering in Flint Weapons 

 It is now some years since I first noticed the fact that in a 

 number of flint weapon heads in my possession a distinct spiral 

 could be traced in the form, this being evidently due in part to 

 the direction of the line of fracture in the flint, but also in part 

 to an exaggeration of this by the hand of the workman. In the 

 last number of the Scientific American is d picted an arrow-head 

 with the edges very distinctly feathered, so that if the weapon 

 witli which it was armed was propelled with any great rapidity, its 

 revolution would be a matter of necessity and would result in a 

 greater steadiness in its line of trajectory. 



After having ascertained that my own weapons were all twisted, 

 I examined a number of others with the view of ascertaining if 

 the same spiral existed in them, and in all I found that there was 

 something lilce it, and the more finish they presented the more 

 twisted they were. 



A very simple method enabled me to show the twist well. I 

 pressed a flint between two pieces of greased pipeclay, then re- 

 moved it carefully and filled its place with liquid plaster of Paris. 

 Cross-sections of this cast in various directions showed the twist to 

 perfection, and I found that the two wings of the flint were 

 twisted in opiiosite directions though identical in relation to the 

 axis of rotation), and that the curvatures were identical with those 

 seen in tlie iron arrow-heads provided with wings which are used in 

 many savage countries to this day, and were till lately, if indeed 

 they are not still, made in large quantities in Birmingham. The 

 most perfectly twisted stone arrow-head which I have yet seen is 

 one made of quartz, where the line of fracture could not help the 

 manufacturer in the least, and where it must have been the result 

 of deliberate workmanship. It was an American weapon. The 

 line of fracture of flint always gives a more or less pronounced 

 spiral, and this may be one of the many reasons for its having 

 been almost universally selected as the material for arrow-heads 

 when it could be got. In fact, it is a difficult thing to find a flint 

 flake of any size which has not a very evident spiral form, and I 

 have a photograph in my possession of two weapons which I 

 have examined and wliich are almost identical, one found without 

 its shaft near Bridlington, in Vorksliire, and one with its shaft 

 found in the hands of a native of New Zealand ; and it would be 

 impossible to tell, from the style of manufacture, which weapon 

 belonged to which country. It is impossible to regard this as 

 mere coincidence, but we must look on it, in each case, as 

 an independent discovery of the principle of the rotation of the 

 rifled projectile. Lawson Tait 



LOCALISATION OF FUNCTIONS IN THE 

 BRAIN 



AT one of the last meetings of the Royal Society, Dr. 

 Burdon-Sanderson related the results of experi- 

 ments he had recently made with a view to the further 

 investigation of the important discovery of Hitzig and 



Fritsch, that there are certain spots on the surface of the 

 cerebral hemispheres by the e.\citation of which the 

 muscles of the opposite side of the body can be thrown 

 into combined action. 



It is well known that Dr. Ferrier, of King's College, 

 who has studied the topographical distribution and limita- 

 tion of these active spots or areas with great minuteness 

 on a considerable variety of animals, has founded upon 

 his experiments a theory that these spots correspond to 

 organs situated at or near the surface of the hemispheres, 

 and that it is the function of these organs to originate 

 combined voluntary movements. Dr. Ferrier has accor- 

 dingly proposed to call them "motor centres." 



As, however, the facts appeared to Dr. Sanderson to 

 be quite as consistent with the view previously entertained 

 by physiologists that the function of co-ordinating volun- 

 tary movements is localised lower down in the cerebro- 

 spinal centres, he thought it necessary to ascertain, with 

 reference to some of the most characteristic combined 

 movements produced by stimulation of the surface of the 

 brain, by the interrupted voltaic current (Hitzig and 

 Fritsch), or by induced currents (Ferrier), whether the 

 very same combinations of inovements could not be 

 produced after ablation of the grey substance in which 

 the " centres " for their production were supposed to be 

 contained. If it could be shown that after complete 

 removal of the " centres," the effects to the production of 

 which they were supposed to be essential could still be 

 observed, this would go far to prove that the facts had 

 been misinterpreted ; and if it could be further shown, not 

 only that the phenomena might present themselves in 

 animals deprived of the centres from which they were 

 supposed to originate, but that they could be produced 

 in such animals by the same methods and under the same 

 circumstances as in normal animals, this would go far to 

 negative the existence of any organs at the surface of the 

 brain to which the term " motor centre " could with any 

 propriety or accuracy be applied. 



In accordance with these considerations, Dr. Sanderson 

 planned experiments, in some of which the superficial 

 convolutions containing " centres " were removed, while in 

 others the whole of the anterior part of the left hemi- 

 sphere as far down as the outer portion of the corpus 

 striatum was taken away with the aid of a sharpened 

 spoon. In each case it was found (i) that when after the 

 removal of the cortical grey substance, the cut surface of 

 white substance is excited by induced currents, move- 

 ments of the opposite side of the body are produced, 

 which are of the same character as those which result 

 from excitation of the natural surface ; (2) that the excita- 

 bility is limited to certain spots, which can be as sharply 

 defined as those demonstrable on the natural surface ; 

 and (3) that the relative positions of the active spots on 

 the cut and natural surfaces respectively correspond 

 closely with each other. 



Simultaneously with the publication of Dr. Sanderson's 

 communication, a paper appeared in Eckhardt's Bcitriigc, 

 in which an account \va:i given of very similar experi- 

 ments, of which the results, though incomplete, corre- 

 sponded, so far as theory w-ent, with those above related. 

 We learn also that Prof. Hermann of Ziirich has also 

 made experiments which have led him to reject in the 

 most unequivocal manner the conclusions of Hitzig and 

 Fritsch. 



THE FORM OF COMETS* 

 II. 



LET us see what ideas, what explanations have been 

 suggested by the aspect of these monstrous pheno- 

 mena, so evidently subject to the influence of the sun. 

 On examining comets, the first idea which is pre- 

 * CoDtlnued from p. 229. 



