532 



NATURE 



[January 22, 1920 



Power from the Sun. 



With reference to Mr. A. S. E. Ackermann's letter 

 in Nature of January 15, in which he states that, in 

 putting the possible efficiency of obtaining power from 

 the sun with the heat engine at less than 2 per cent., 



1 have used too low a figure, I may point out that, 

 whereas Mr. Ackermann's figure of 432 per cent, was 

 a maximum obtained presumably under specially 

 favourable conditions, and as I understand in Egypt, 

 in suggesting a figure of less than 2 per cent. I was 

 referring to what could be expected " in this latitude 

 and in this climate " — that is to say, in England, and 

 also as an average during the hours of daylight 

 throughout the whole year. 



For the purpose of my argument, and in comparison 

 with the very much higher efliciencies that are 

 theoretically possible if the radiation can be directly 

 utilised without first turning it to heat, with the con- 

 sequent avoidance of the second law of thermo- 

 dynamics, I do not think that the difference between 



2 and 4 per cent, is of much importance ; but, even so, 

 I should be surprised to learn that Mr. Ackermann 

 would e.xpect to obtain an efficiency of even 2 per cent, 

 anyvi-here in England throughout the year. 



.\. A. Campbell Swinton. 

 66 Victoria Street, S.W.i, January 17. 



Sedimentation of Blood Corpuscles. 



I HAVE noticed lalely that if oxalatcd or defibrinated 

 blood is put to stand in narrow tubes, the corpuscles 

 sediment a good deal faster if the tube is inclined 

 than when it is vertical. Thus with tubes about 

 2-7 mm. internal diameter there were, after 20 hours, 

 4. 23, 35, and 42 per cent, of clear serum with tubes 

 inclined at 0°, 22^°, 45°, and 671° respectively. In 

 another rough experiment with tubes of different 

 diameters, all filled to a height of 40 mm. with diluted 

 blood, after 5 hours there were the follov^ing pro- 

 portions of clear serum : — 



The phenomenon seems to depend on the vertical 

 height of the columns of blood, and it occurs to me 

 that the slight Brownian movement of the lower 

 corpuscles may interfere with the sedimentation of 

 those above. But I should be glad if someone would 

 tell me the explanation : the phenomenon is perhaps 

 well known in some other form. A. E. Boycott. 



Medical School, University College Hospital, W.C. 



The Einstein Theory and Spectral Displacement. 



One of the "crucial phenomena " in connection with 

 the Einstein theory is the displacement of the spectral 

 lines towards the red when the emitting atom is in 

 a position where the gravitational potential is large. 



In the case of the sun this displacement is so small 

 that its existence is a matter of doubt. But the 

 amount of the displacement varies as the mass of the 

 sun or star concerned, divided by its radius, and in the 

 case of giant stars, such as Canopus, Arcturus, or 

 Antares, should give a result corresponding to a reces- 

 sion of many hundreds, if not thousands, of kilometres 

 per second, whereas, in fact, these stars show no 

 abnormal radial velocities. 



It may be pointed out that the effect varies as the 

 product of the area and density, factors as to which 

 NO. 2621, VOL. 104] 



the magnitude and spectrum of a star enable astro- 

 nomers to make a fair approximation, at any rate 

 as to minimum values.* 



These facts must, of course, have been considered 

 by the supporters of the theory, and I think that an 

 explanation would be interesting and useful. 



H. Fletcher Moulton. 



II King's Bench Walk, Temple, E.G. 



Mr. Fletcher Moulton is quite correct in 

 stating that the shift of the spectral lines varies as 

 mass/radius, but his expectation of spectral shifts 

 measured by hundreds or thousands of kilometres per 

 second does not appear to be justified. All the evi- 

 dence available, deduced from visual binaries, Algol 

 variables, and spectroscopic binaries, points to the 

 conclusion that the masses of the stars vary between 

 much narrower limits than their brightness. We have 

 no clear evidence of any star having a mass so great 

 as forty times that of the sun ; moreover, the most 

 massive stars known to us are apparently in a much 

 more diffused state than the sun, so that the ratio of 

 spectral shifts is much less than that of masses. 



We cannot use individual stars to test the Einstein 

 effect, for we do not know the radial motion inde- 

 pendently of the spectroscope, as we do in the case of 

 the sun. All that we can do is to take the mean of .1 

 large number of spectra and see whether there is a 

 systematic shift towards the red ; such a shift does 

 exist, and the difficulty is rather that it is too large 

 than too small to ascribe wholly to the Einstein effect. 

 Thus Campbell ("Stellar Motions," p. 199) says : "Of 

 Type II. stars (that is, F5 to M), 371 have positive 

 velocities and 352 negative. Of Type I. stars (that is, 

 O to F4), 215 have positive velocities and 122 nega- 

 tive." Subdividing further, he gives the following 

 mean velocities of recession in km. per second : B to 

 B9, 4-93; A, 018; A2 to F8, 060; G to M, 0-91. Dr. 

 de Sitter, taking the average mass and density of s. 

 B star as 10 and i/io respectively, finds 14 for tht- 

 Einstein effect, about one-third of the observed 

 quantity. 



We do not know the character of the atmospheric 

 circulation in the stars ; this, as well as pressure 

 effects, may well have some influence on the mean 

 results. Taking the stars as a whole, it must be 

 admitted that their verdict, though by no means con- 

 clusive, is, so far as it goes, in favour of Einstein. 

 Andrew C. D. Crommelin. 



Use of a Prismatic Binocular for Viewing Near Objects. 



A FEW years ago, with a view to the observation 

 of close objects out of doors, I procured some glass 

 adapter lenses for use on the object glass of the half 

 of a prism binocular (xi2) which I carried about 

 with me. Finding, however, that this method in- 

 volved the use of several glass adapters, and that 

 with it I had to know the exact distance of my 

 objective, I prevailed on an optician — after lengthy 

 argument, he deeming the experiment impracticable— 

 to remove the eyepiece and lefit it for use with a long 

 screw thread. The result was most satisfactory; by 

 this device I can draw out the eyepiece and adjust it 

 to the proper distance for any observation down to four 

 feet off. This device is also very useful for indoor 

 work, such as observation on the occupants of an 

 aquarium. 



The device may be useful to other observers, who 

 will find that the necessary alteration can be easily 

 made. D. Wilson Barker. 



, Flimwell. 



