276 



NATURE 



[April 29, 1920 



the actual source of light upon the object under 

 examination) is carefully examined. The well- 

 arranged and conclusive experiments described by the 

 author lead to an unconditionally negative answer 

 which will not surprise those who are familiar with 

 the theory of microscopic image-formation, but the 

 results, being experimental, may put an end to the 

 barren discussions on this subject by practical micro- 

 scopists. A very neat and compact arrangement for 

 the efficient and perfectly controlled illumination of 

 objects in accordance with the results of the inves- 

 tigation is described. Some of the theoretical views 

 in the first p^rt of the paper are not acceptable. 

 Whilst it is true that the usual methods of illumina- 

 tion do not strictly realise the assumptions under- 

 lying Abbe's theory, it is surely not open to question 

 that the theoretical work and the rare theoretical 

 calculations of images have always been carried out 

 in accordance with the theory. The statement that 

 if the ideals of critical illumination were realised, then 

 resolution would be destroyed, is quite untenable, 

 for as that ideal is to make the object behave as if 

 it were self-luminous, the statement amounts to 

 claiming that a self-luminous object — say a white-hot 

 one — could not give a distinct image, which is absurd. 



We have received the fourth report (for 1916) of the 

 seismological observations at De Bilt, Holland 

 (Konink. Nederl. Meteor. Inst., No. 108, 1918, 

 pp. 1-102), in which are given full details of the 

 records obtained from horizontal motion seismographs 

 of Galitzin and Wiechert and a pair of Bosch hori- 

 zontal pendulums, as well as a summary of the 

 results from other observatories of the more important 

 earthquakes. From this report we learn that the 

 munitions explosion atFaversham on April 2, 1916 

 (see Nature, vol. xcix., 1917, p. 250), was registered 

 by the Wiechert and Galitzin pendulums at De Bilt. 

 The effects of the air-waves of this explosion were 

 widely manifested in Holland, especially in the 

 western districts, by the shaking of doors, windows, 

 and pictures, as if by a slight earthquake. 



One of the strongest earthquakes felt in Porto Rico 

 since the European occupation occurred on October 11, 

 19 18, the official Report of the Earthquake Investiga- 

 tion Commission (Washington, 1919), by Prof. H. F. 

 Reid and Mr. S. Taber, having recently been pub- 

 lished. The approximate position of the epicentre is 

 given as 18° 30' N. lat., 67° 20' W. long., in the 

 north-east portion of Mona Passage, and the time 

 of occurrence, within a very few seronds, at 

 2h. 14m. 38s. p.m. (Greenwich mean time). The 

 earthquake was followed after a few minutes by a 

 sea-wave which reached a height of about 4J metres 

 above mean sea-level along the north-west coast of Porto 

 Rico, the first movement of the water, wherever seen, 

 being one of withdrawal. The epicentre lies along a 

 deep submarine valley, the slopes of which are so 

 steep that they must be regarded as the result of 

 faulting. During the last half-century the north-west 

 coast of the island has been noticeably subsiding, and 

 the authors attribute the earthquake and sea-wave 

 to a vertical displacement near the head or on one 

 side of the submarine valley. 

 NO. 2635, VOL. 105] 



Our Astronomical Column. 



Eclipse of the MooN.-^There will be a total 

 eclipse of the moon on the night of May 2-3. The 

 following are the Greenwich times of the various 

 stages : — 



Moon enters penumbra ... Maya, 10.49p.m. 



Moon enters shadow ... ... ,, 12.0 mid. 



Beginning of total eclipse ... ,, 3, 1,15 a.m. 

 End of total eclipse ... ... ,, 2.27 



Moon leaves shadow ... ,, 3.41 



Moon leaves penumbra ... ,, 4.53 



At Greenwich the moon rises on May 2 at 7.5 p.m. 

 and sets on May 3 at 4.34 a.m. The whole of the 

 phases of the eclipse will thus be visible. 



During some lunar eclipses the disc of our satellite 

 has remained fairly bright, while at others it has 

 been scarcely visible. If atmospheric conditions are 

 favourable, observations of the character of the 

 shadow on this occasion might be made and, possibly, 

 photographs taken. During the lunar eclipse of 

 July 4, 19 17, observations made at the Bordeaux- 

 Floriac Observatory showed that throughout totality 

 the north edge of the moon seemed brighter than 

 the south. 



Mars and Wireless Signals. — It is regrettable that 

 in these days, when results of great interest concern- 

 ing solar and stellar physics are continually being 

 reached, the public should have its attention concen- 

 trated upon sensational assertions, such as the Porta 

 planetary scare last December and the suggestion of 

 wireless signals to or from Mars which is now under 

 discussion in the daily Press. Leaving aside the 

 physical difficulties of such communication — which, 

 though considerable, are perhaps not insuperable— a 

 very little consideration suffices to show the utter im- 

 probability, closely approaching to impossibility, that 

 the idea of signalling should be mooted simultaneously 

 on the two planets. On any view as to the develop- 

 ment of the planetary system, the periods when Mars 

 and the earth pass through corresponding stages 

 would be likely to be separated by millions of years. 

 The suggestion that the Martians have kept up the 

 practice of signalling at every opposition through 

 such a period as this, in the patient hope that they 

 might one day be answered, makes too strong a 

 demand upon our credulity. 



The April Meteor Shower. — ^The weather was 

 moderately fine at the time the Lyrids were expected, 

 and a fair number of them were visible. The best 

 night seems to have been that of April 21, when the 

 sky was generally clear and the maximum abundance 

 occurred near midnight. The phenomenon was ob- 

 served by Miss A. Grace Cook at Stowmarket, Mr. 

 S. B. Mattey at Plumstead, S.E., Mr. C. P. Adamson 

 at Wimborne, Mr. W. F. Denning at Bristol, and 

 others. The Lyrid meteors formed about one-half of 

 the total number visible on the nights of April 19, 20, 

 and 21, and nearly all of them left streaks. They 

 moved with moderate velocity, being decidedly slower 

 than either the Leonids or the Perseids. As regards 

 brightness they were much above the average, and 

 some fine ones were recorded on the dates mentioned. 



The Wasting of Stellar Substance. — This is the 

 title of a paper by Prof. F. W. Very in Scientia for 

 April. It will be remembered that Prof. Eddington 

 made the suggestion in the Observatory last Septem- 

 ber that the immense duration of the radiation from 

 the stars might be explained by the annihilation of 

 some of their component atoms through collision, and 

 the consequent liberation of their stores of energy. 



