May 19, 1 9 10] 



NATURE 



339 



A Difference in the Photoelectric Effect caused by 

 Incident and Divergent Light. 



In a letter dated April 26 which appeared in Nature of 

 May 12, Mr. Stuhlmann, of Princeton University, U.S.A., 

 describes some experiments which he has carried out on 

 the photoelectric effect of incident and emergent light. I 

 should like to mention that I have been carrying out some 

 experiments on the same subject at the Cavendish Labora- 

 tory, Cambridge, and obtained the same efifect as that 

 described quite recently by Mr. Stuhlmann. The experi- 

 ments were completed more than two months ago, and 

 the results obtained described in a paper communicated 

 by Sir J. J. Thomson to the Royal Society on March 25. 

 In view of the appearance of the above letter they may 

 be briefly described here. 



A thin quartz plate was covered with a very thin film 

 of platinum in a discharge tube by directing the discharge 

 from a platinum kathode on to it. The kathode radiation 

 per unit time from the film under the influence of ultra- 

 violet light was measured (i) when a constant beam of 

 ultra-violet light was incident at right angles to the film ; 

 (2) when the beam emerged from the film, passing in this 

 case first through the quartz plate. The intensities of the 

 kathode radiations were found to be as i to i-i6, while 

 the intensities of the incident and emergent beams were 

 as I to 0-5. The conclusion that can be drawn from the 

 experiments is that an electron liberated by ultra-violet 

 light has a component of motion in the direction of pro- 

 pagation of the exciting light. 



Cambridge, May 14. R. D. Kleeman. 



Steam Tables. 



Is Natcre of April 21a review appeared of Profs. Marks 

 and Davis's excellent new tables of steam properties, in 

 which it is stated, without qualification, that the new 

 calculations of tlve total heat of saturated steam are based 

 upon a second-degree equation H = a+&t + c<'. Both in 

 the explanatory notes to the tables, and still more emphatic- 

 ally and repeatedly in a paper printed in the Proc. Am. 

 Acad. Arts and Sciences, March, 1910, the authors state 

 that this equation does not apply outside the limits 

 20o°-40o'' F. Simple numerical tests also., prove that the 

 tabular figures do not agree with this formula outside these 

 limits, and the formula would give H its maximum value 

 at 723° F. higher temperature, and four heat units more 

 in quantity, than the tables make it. Mr. Davis says that 

 no formula yet discovered will apply throughout the full 

 range, and above about 450° F. the figures given are not 

 credited with a high degree of accuracy or certainty. 



Basing upon these new tables, I constructed a formula 

 for total heat, which was published on December 24, 1909, 

 in the Engineer, and gives the tabular results with prac- 

 tical exactitude from 70° to 500° F., that is, from 036 

 to 684 lb. per sq. inch absolute pressure. This formula is 



H =1826 4-*- 10' -f 8(1620- 0- 

 The following are its '* errors " as compared with Marks 

 and Davis's tables : — 



/" F. : 30 

 H Diff. -2-7 



H Diff. ! 



50 60 70 80 



-I "6 —I "2 -o'7 -o'S 



250 



-0'2 



300 



-O'l 



350 400 

 +o"3 +o'i 



150 200 



+o'4 — o-i 



450 500 600 



-0-4 +0-9 +249 



The order of accuracy aimed at in this formula is further 

 illustrated by the factors 0-9938, 0-997, 10066, and 1055 

 having been tried for the term in t instead of 1, and having 

 failed; while, in place of 10' -r 8 =1,250,000, one of the 

 factors which was tried and failed was 1,251,150. 



The maximum value of H given by this formula is 1210, 

 which is identical with that of the tables, but it occurs at 

 502° instead of 480°. Exactitude in placing this tempera- 

 ture of maximum H by the purely graphic analysis of a 

 ver\- few experimental results in its neighbourhood which 

 was used by Marks and Davis, is evidently impossible. The 

 tables do not venture to give any values of H above 6<x)° F. 

 My formula may very likely gjve considerable errors near 

 the "critical point," which is somewhere near 690° F. 

 Here other physical influences probably become prominent, 

 as also, very probably, at low temperatures near that of 

 maximum water density. Robert H. Smith. 



3 Thirlmere Road, Streatham, S.W., May 2. 



NO- 21 16, VOL, S}] 



I .NOTICED at the time of its publication in the Engineer 

 Prof. Smith's communication of the discovery of an 

 empirical formula which would represent the values of the 

 total heat even more accurately than that of Messrs. Marks 

 and Davis. When speaking of their own formula the 

 authors remarked (pp. loo-i) : — ^" It has been used for 

 the range above 2x2 in these tables"; but they evidently 

 meant to limit the range to 400° F., although this is not 

 clearly expressed in the paragraph from which the above 

 extract is taken. 



I agree with Prof. Smith that it is too much to expect 

 any empirical formula to predict what will occur at the 

 " critical point." The Reviewer. 



Fireball in Sunshine. 



O.N May 10, at 7h. 52m. a.m., a magnificent meteor 

 was seen by many obser\ers in the Midlands. I have read 

 a considerable number of descriptions of the object, but 

 they are not very definite. The meteor was witnessed by 

 persons not well versed in astronomy and exact positions 

 for the apparent flight. It was a brilliant object w^ith 

 a bluish nucleus and tail of red sparks ; the observed 

 velocity was moderate. Though the sun was shining 

 the meteor shone with conspicuous effect, and more than 

 one person supposed it to be Halley's comet, or, at any 

 rate, a fragment of that body. 



Seen from Birmingham, the meteor's path was from 

 the north-east to north-west, and one good obser\'ation 

 ascribes to it an altitude of 30 degrees in a perfectly hori- 

 zontal course. It is difficult to assign the real path, but 

 an approximate computation places the height at from 

 about 83 to 32 miles along a luminous trajector}' of nearly 

 100 miles at a velocity of 20 miles per second. The posi- 

 tion of the radiant point is doubtful, but several of the 

 observations indicate it in Auriga or Perseus. The meteor 

 travelled over the region of Yorkshire or Lincolnshire 

 towards the district of Liverpool, but in the absence of 

 more exact materials it is quite impossible to derive the 

 path with certainty. 



No stars being visible in the bright blue of the May 

 morning which presented this unusual celestial pheno- 

 menon, the observers could not locate, the position with 

 the required accuracy ; but it is hoped that further obser\'a- 

 tions will come in from the northern counties of England. 

 The *' daylight fireball " of May 10 last reminds us of a 

 similarly brilliant object which flashed out amid the sun- 

 shine on October 6 last at 9.40 a.m. W. F. Desxixg. 



Observations of Halley's Comet and Venus. 



It may interest readers of Nature to know that the 

 planet \'enus was visible — plainly visible — in Natal all day 

 to-day up to the time of its setting. The air was wonder- 

 fully clear and free from dust or moisture. At four o'clock 

 in the morning Venus was unusually brilliant, the light 

 therefrom shining into my bedroom. Halley's comet rose 

 above the horizon at about 4.30, and, although distinctly 

 visible to the naked eye, was pale and insignificant com- 

 pared to the planet. By six o'clock the comet was no 

 longer visible, having paled away before the sun had 

 actually risen. .At mid-day excited groups of natives and 

 Europeans were gazing with wonder at what was mis- 

 takenly considered to be Halley's comet visible in broad 

 daylight ! Venus was then in the zenith, her glory defy- 

 ing the power of the mid-day sun. E. T. Mullens. 



Pietermaritzburg, Natal, April 22. 



Earwigs of India. 



In Nature of April 14 was published a review of my 

 half-volume on the Dermaptera in the " Fauna of British 

 India " series, in which the reviewer directed attention 

 to a most regrettable oversight on my part in omitting to 

 allude to the British Museum when acknowledging the 

 various sources which supplied me with material. 



Fortunately, the frequent references .in the text betray 

 my indebtedness, but I should be glad to take advantage 

 of the hospitalit>- of your pages to make amends, at the 

 same time thanking your reviewer for pointing out this 

 extraordinary omission, by expressing now my apprecia- 

 tion of the invariable and well-known courtesy of my good 

 friends among the oflicials of the museum. 



Eastr>', Kent, May 5. Malcolm Burr. 



