174 



NA TURE 



[June 22, 1905 



occupy the first piace for some time to come, so far as 

 elementary education is concerned. 



The second lecture deals with the work of Maggi 

 and the morphology of the cranial bones. This is a 

 subject on which Dr. Frassetto has himself published 

 some valuable studies. He would have done well to 

 indicate in his lecture that some, at any rate, of the 

 new views on the number of centres of ossification are 

 based on what seems to be an unduly small collection 

 of cases. The third lecture treats of de Giovanni 

 and his work in clinical anthropology, which deals 

 with a patient according to his morphological charac- 

 teristics rather than as an individual. Finally, we 

 have a sketch of the work of Lombroso on criminality 

 and genius. Dr. Frassetto insists on the need for 

 scientific treatment of criminals, especially those of the 

 habitual class. 



It goes without saving that in brief studies of this 

 sort we onlv find the broad outlines, without qualifi- 

 cation or hint of difficulties, and herein lies perhaps 

 a certain danger for the unfledged anthropologist who 

 attends the academic courses. The book is, however, 

 readable, and offers an example to English anthro- 

 pologists who wish to ii terest a larger public. 



N. W. T. 

 Catalogue of the Lepidoptcra Phalaenae in the British 



Museum. Vol. v. Catalogue of the Noctuidse in 



the Collection of the British Museum. By Sir 



George F. Hampson, Bart. Pp. xvi + 634; pis. 



Ixxviii-xcv. (London : Printed by Order of the 



Trustees, 1905.) 

 We congratulate the authorities of the British Museum 

 and the indefatigable author on the steady progress 

 of this important work, of which a fresh volume 

 appears, with almost clockwork regularity, every two 

 years. The present volume is the second devoted to 

 the Noctuidas, and contains the second subfamily, the 

 Hadeninae. These are much less showy moths than 

 those dealt with in the first three volumes of the 

 series, and are more subdued in their colouring ; but 

 they are perhaps more interesting to British entomolo- 

 gists, for the family is fairly well represented in the 

 northern hemisphere, although in a work devoted to 

 the moths of the whole world, British, or indeed 

 European, species are few and far between. The 

 work is profusely illustrated, the descriptions are full 

 but not too lengthy, and short notices of larvae, where 

 known (some of which are here published for the first 

 time), have been included. The keys to the genera 

 and the tables of species will also be found very useful 

 by working entomologists. A table of the phylogeny 

 of the 78 genera into which the author divides the 

 Hadenin» is given on p. 2, but without comment, 

 which we think is wise, for such tables, in the pre- 

 sent state of our knowledge, can only be tentative ; 

 and comments on the supposed affinities of genera 

 have often a tendency to become too dogmatic. 



Svnonymv cannot, of course, be given in full in a 

 work of this character, but in the case of European 

 species, which are most burdened with it, the neces- 

 sity for further details is largely obviated by a 

 reference to .Staudinger's last catalogue ; still, we 

 think that, in the case of the few British species, 

 Barrett's " Lepidoptera of the British Islands " might 

 have been referred to. 



We heartily commend this important book to the 

 working entomologists of all countries. Five volumes 

 have already appeared, but if it is ever completed it 

 will certainly far exceed in bulk the twenty-se\-en 

 volumes of the " British Museum Catalogue of Birds." 

 Hitherto it has been wholly the work of one man, and 

 we hope that when he finally lays down his pen, a 

 very large proportion of the gigantic task of describ- 

 ing the moths of the world will have been accomplished 

 by his hands. 



NO. i860, VOL. 72] 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for tliis or any other part of Nature. 

 No notice is taken of anonymous communications.] 



The Spinthariscope and Retinal Excitability. 



I H.AVE recently been making a series of observations 

 upon retinal excitability, and have used, among other test 

 stimuli, the well known flashing scintillations of a pocket 

 spinthariscope. The special value of the instrument in 

 this connection is the subminimal or minimal intensity of 

 the retinal excitation judged by the disappearance or 

 appearance of visual sensation. The method used by 

 physiologists for ascertaining whether any given condition 

 alters the excitability of a tissue is that of stimulating 

 periodically the tissue by subminimal or minimal exciting 

 agencies under constant conditions and then changing one 

 of these conditions ; if, as the result of such change, the 

 subminimal or inadequate stimulus becomes adequate, the 

 excitability of the tissue has been raised by the change ; if. 

 on the other hand, the minimal or adequate stimulus 

 becomes inadequate, then the excitability of the tissue has 

 been lowered by the change. 



It is well known that with the ordinary pocket spinthari- 

 scope no luminous effects are seen unless the eye has been 

 rendered sufficiently sensitive by some minutes' darkness; 

 this is especially the case during the daytime, the effects 

 at night being almost instantaneously visible. The stimu- 

 lation of the retina by the scintillating flashes is thus of 

 the minimal order, and becomes subminimal when the eye 

 is exposed to daylight. 



It is thus possible to place the eye under different con- 

 ditions, and to determine by means of the visibility of the 

 flashes in the spinthariscope whether the retinal excitability 

 has been raised or lowered ; the method has the merit of 

 great simplicity, all that is necessary being to go into a 

 dark room and immediately look through the instrument ; 

 the time necessary for the appearance of the first visible 

 luminosity and for the full appearance of the flashes is 

 longer the lower the general excitability of the retina. 



A further point of physiological interest is brought out 

 by simple experiments along these lines. It is well known 

 that when the eyes at night look at groups of stars, faint 

 groups not in the direct line of vision are distinctly seen 

 which disappear when the gaze is directed towards them. 

 There is an accumulating mass of evidence that this 

 familiar experience is the sensory aspect of a modified con- 

 dition of the retina, the modification consisting in an 

 augmented excitability of the peripheral portions of the 

 retina. It appears probable that such peripheral augmented 

 excitability is localised particularly in the outer segments 

 of one set of retinal elements, the rods, which contain 

 the visual purple discovered by Kiihne. The rods are 

 extremely numerous in the peripheral region, and con- 

 stitute the sole elements in nocturnal birds such as the 

 owl. The visual purple of the rods is blanched by light, 

 especially by the more actinic rays, but the blanching dis- 

 appears with darkness, and this re-constitution of the 

 substance is associated with the presence of the choroidal 

 pigment. There is thus an adaptation process which 

 renders the dark-adapted eye more excitable than it other- 

 wise would be, and this augmented excitability is especially 

 prominent in that part of the retina which contains large 

 quantities of rods, viz. the peripheral portions. The 

 specialised elements of the centra! part of the retina 

 (macula lutea) consist in man almost entirely of cones ; 

 it is undoubted that in daylight this part is the most 

 excitable region, and that it possesses to a remarkable 

 degree the capacity of localised response, thus enabling 

 two sources of light to be discriminated as distinct when 

 so near .together that they subtend an extremely small 

 angle. At night, or with the dark-adapted eye, the w-holc 

 condition is modified, and the peripheral part of the retina 

 has its excitability augmented more than the central part, 

 so that sources of light of subminimal intensity for the 

 latter are adequate to excite the former ; these facts are 

 readily demonstrable by means of the spinthariscope. 



Thus if in the day time the observer takes the spin- 



