December 8, 192 1] 



NATURE 



469 



'•comes useless for exact work, and we long ago tried 

 ind discarded such a method for this reason. 



A method that does not possess this disadvantage 

 is to use partially platinised glass screens. These can 

 asily be made by methods already known, and the 

 ifect on any illuminant is to reduce its intensity with- 

 out changing its character. In my own practice I 

 have a number of these, the absorption of which has 

 been tested by a photometer, mounted in a small 

 frame so that they can in a moment be passed along 

 in the path of the beam. The process is, in fact, less 

 troublesome than operating a sliding resistance. The 

 'ight can therefore be reduced by a known amount at 

 ich step. The advantage of this when a change is 

 ade, say from a low power ocular to a high one, is 

 !)V'ious. The relative light-transmitting power of the 

 culars is known, and it is, therefore, only necessary 

 > move a suitable screen into position in each case 

 .hen the intensity of illumination in the field of view 

 i> the same for both. Should a gradual change of 

 intensity be required, then an arrangement recently 

 placed on the market by Messrs. R. and J. Beck, Ltd., 

 will do all that is necessary. It consists of two 

 graduated neutral wedges which are moved across one 

 another by a simple gearing so that any degree of 

 opacity may be obtained and the light controlled 

 between wide limits. J. E. Barnard. 



National Institute for Medical Research, 

 Hampstead, London, N.W.3, November 29. 



Hybridity and the Evolution of Species. 



Dr. J. P. LoTSY, in his letter to Nature of 

 November 24, asks zoologists to answer the question : 

 Is there any evidence that the presence of oligopyrene 

 and apyrene sperms in some insects and molluscs is 

 due to hybridity? 



In the first place the atypic sperms of molluscs are 

 not of the same nature as those of moths, for in the 

 case of moths the appearance of atypicality is during 

 the maturation stages. The spermatocyte which will 

 give rise to the atypic sperms has not yet been dis- 

 tinguished cytologically from one which will give rise 

 to normal sperms. 



So far as the atypic sperms of prosobranch mollusca 

 are concerned, quite a different condition holds good. 

 Some years ago it was shown (Quart. Jour. Micr. 

 Science, vol. 63, p. 421) that apart from the then 

 known fact that the atypic and typic spermatocvtes 

 were cytologically distinguishable, it was possible to 

 trace back these two sorts of cells to two different 

 kinds of germinal epithelial cells. In the primary 

 spermatogonium of the atypic series, the mitochondria 

 were granular, while those of the typic series were 

 sausage- or rod-shaped — a difference which I showed 

 to hold good through growth stages and maturation 

 divisions. It seems clear that the atypicality of the 

 sperms of some molluscs is a qualitv deep-seated in 

 certain germ-cells, while that of some moth sperms 

 is possibly merely traceable to abnormalities in meta- 

 bolism due to the rapidly changing conditions during 

 histogenesis. 



Now with regard to Dr. Lotsy's quer\- as to 

 •whether the atypicality of mollusc sperms is evidence 

 as to hybrid ancestry, it may be mentioned that in 

 Pulmonata Mollusca, the mitochondria are always 

 granular in the spermatocytes, while the atypic 

 spermatocytes of prosobranch Mollusca alone are 

 granular, while those of the tvpic are sausage-shaped. 



I leave an interpretation of this important fact to 

 Dr. Lotsy or his opponents. One more word — these 

 differences in the mitochondria of the two kinds of 



spermatocytes of Paludina can be seen intra vitam 

 in freshly teased out cells. 



J. Bronte G.atenby. 

 University of Dublin, November 26. 



A Simple Micro-barograph. 



Readers of Nature may be interested in a simple 

 form of differential barometer by means of which 

 changes in air pressure as small as one part in a 

 hundred thousand may be readily observed. The 

 apparatus consists of a vacuum flask to the mouth of 

 which is fitted a two-holed cork. One hole bears 

 a capillary tube and the other a small tube provided 

 with a tap. 



A small drop of liquid is introduced into the capil- 

 lary, and with the tap B open, it can be made to 

 occupy any desired initial position. Tap B is now- 

 closed, and the movements of the drop A following 



?= A 



Fig. I. 



the changes in volume of the enclosed gas will indi- 

 cate very small changes of pressure. The capillan,", 

 of course, must be kept level during the observ'ations. 



Using a vacuum flask of capacity 450 c.c, a capil- 

 lary of 0-3 sq. mm. cross-section, and a drop of light 

 paraffin oil as the liquid, the writer was able to 

 demonstrate the changes of atmospheric pressure 

 which occur during a change of vertical height of 

 I ft. _ 



The instrument was primarily designed for the 

 purpose of demonstrating change of atmospheric 

 pressure with height, and should be of use to teachers 

 to demonstrate this phenomenon. 



It would be interesting to know whether such an 

 instrument has other applications. It may prove of 

 interest to meteorologists for observing minute changes 

 of atmospheric pressure during small time -intervals, 

 and possibly also to aviators if set up in a less sensi- 

 tive form. A. Wechsler. 



Hackney Technical Institute, London. 



NO. 2719, VOL. 108] 



Scientific Workers in Russia. 



Many British scientific workers have acquaintances 

 and friends among Russian men of science, and as 

 Christmas is approaching they may wish to send a 

 Christmas greeting which will support their colleagues 

 during the hardest time that Russian science has ever 

 had to endure. The American Relief Administration 

 (67 Eaton Square, London, S.W. i) receives contribu- 

 tions in money, and from its famine relief stores in 

 Russia will guarantee to deliver a parcel of food- 

 stuffs to any person designated and will forward a 

 receipt from the latter. 



If a difficulty is found in selecting a particular 

 Russian scientific worker, the parcel may be addressed 

 simply to the Rector of the University of the citv in 

 question or to the President of the Military Medical 

 Academy at Petrograd or the Academy of Sciences 

 for distribution among the men of science of the 

 respective institutions. 



If necessary. I can furnish information about a 

 number of Russian scientific workers. 



V. KORENCHEVSKY, 



Lister Institute of Preventive Medicine, 

 Chelsea, London, S.W.i. 



