September 14, 1905] 



NA TURE 



495 



this asteroid had a magnitude of 11-3, and on August 31 

 Dr. Palisa recorded it as being of the eleventh magnitude ; 

 at iih. 44m. (Vienna M.T.) on September 5, however, it 

 had sunk to tlie twelfth. 



The position of this body at iih. 39-9m. (Kbnigstuhl 

 M.T.) on August 23 was R.A. = 22h. 37-9m., dec. = —7° 55', 

 and on September 5d. iih. 4-4m. (Vienna M.T.) 

 R.A. = 22h. 27m. 47-35., dec. = —9° 5' 45". 



Interpret.mion of Spectroheliograph Pictures. — In 

 No. 4044 of the AsUonomische Nachrichten, M. N. Donitch 

 discusses the results obtained by Messrs. Hale and Eller- 

 man, regarding the different chromospheric layers shown 

 on their spectroheliograph negatives, in a new light. He 

 points out that in spectrograms of the chromosphere taken 

 during total eclipses of the sun, the lower layers of the 

 eruptions, i.e. those nearer to the moon's limb, appear to 

 be the most extensive, but in Prof. Hale's photographs 

 (plate v.. No. i, vol. xix., of the .istro physical Journal) 

 the opposite appears to be the case, the higher, less dense 

 layers being more extensive than those near to the photo- 

 sphere. 



This discordance -between the two results is, in the 

 opinion of M. Donitch, only apparent, and may be ex- 

 plained by the suggestions he advances. He assumes that 

 the inequalities on the surface of the photosphere are so 

 small as to be incomparable with those in the layers of 

 calcium vapour which overlay it. Where this vapour is 

 thin it will only produce the ordinary narrow reversal, 

 producing on the negative a calcium area which is at a 

 low pressure, and, therefore, according to Messrs. Hale 

 and EUerman, is situated in the upper regions of the 

 chromosphere. This same reversal is also shown by the 

 vapours, which are, in reality, at a greater elevation, so 

 that, using the monochromatic reversal, one obtains on the 

 photograph the forms of the calcium clouds of which the 

 temperature and pressure are relatively low, whatever may 

 be their elevation above the photosphere. For this reason, 

 as y[. Donitch believes, the first photograph, which shows 

 more extensive areas of calcium vapour, and according to 

 the Yerkes observers represents simply the upper layers of 

 the disturbed areas, really also represents the thinner 

 extensive layers of vapour which are shown on eclipse 

 spectrograms as the broad bases of the eruptions. 



A second photograph taken with the secondary slit set 

 on the broadened H reversal (A = 3q6-2 fifi.) only registers 

 those layers of calcium vapour which, being part of a 

 thick layer, are subjected to a sufficient difference of 

 temperature and pressure to produce the broadening ; and 

 these may, in many cases, be at a greater elevation than 

 the thin layers shown as part of the " calcium " area on 

 the first photograph. 



Similarly in regard to the two photographs shown on 

 plate viii. of Messrs. Hale and EUerman's paper, M. 

 Donitch believes that it is the second, taken with the 

 secondary slit set at A 3968-6, that reveals the general 

 distribution of the vapours in projection, whereas the first 

 only reproduces the higher agglomerations of the vapour 

 which dominates the lower layer. 



The Observatory of Paris. — M. Lcewy's report for the 

 year 1904 is far too lengthy to be reviewed as a whole in 

 these columns, but one or two of the more important 

 details may be mentioned. In his introduction, the director 

 mentions the progress made during the year in the Eros 

 campaign, and also indicates how the photographs of the 

 moon, taken for the large atlas he is preparing, afford 

 evidence that the moon, and, inferentially, the planets, 

 solidified from the surface towards the centre. 



M. Bigourdan has temporarily arrested his observations 

 of nebulae with the equatorial of the east tower in order 

 that the dome and instrument may be prepared for the 

 determination of the absolute constant of aberration by 

 M. Loewy's new method. 



.\ study of the garden meridian circle showed that a 

 difference of o"-45 existed between the readings of the two 

 circles. Various possible causes for this discrepancy were 

 examined, and finally it was discovered that the method 

 of illuminating the microscope wires was at fault. The 

 microscopes have been replaced by others, and the differ- 

 ence thereby eliminated. 



The astrophysical department is awaiting the arrival 



NO. 1872, VOL. 72] 



of apparatus before making celestial observations, but in 

 the meantime M. Hamy has carried out several laboratory 

 researches, the chief of which related to the constancy of 

 wave-lengths in the solar spectrum. He found that when 

 the temperature of cadmium vapour in a vacuum tube was 

 raised about 15° C, in the neighbourhood of 300° C, the 

 line at A 508 diminished several units of the order of 

 I 1J.X10-', and he suggests that the variation of tempera- 

 ture in the solar atmosphere may produce similar results. 



During 1904, 80 catalogue and 31 carte plates were 

 obtained in connection with the carte du del operations, 

 whilst 67 plates containing 16,656 star-images were 

 measured. 



AN ELECTRIC MICROMETER.^ 

 "PHERE is no finality in experimental measurement. 

 In physics it is a common experience for a present- 

 day worker, with better appliances and a wider horizon 

 than his forerunners, to surpass all previous experimental 

 work in accuracy. As knowledge increases it becomes 

 more minutely exact, and nowadays the physicist has often 

 to measure lengths much less than anything visible in 

 any microscope. 



There are various means of measuring small distances. 

 We will take them in order, commencing with the least 

 sensitive : — (i) The unaided eye cannot perceive much less 

 than i/io millimetre. (2) With the aid of the microscope 

 the eye can see as little as 1/5000 millimetre. (3) The 

 measuring machine used for engineering gauges will detect 

 differences of 1/8000 millimetre. (4) By using interference 

 bands of light we can perceive movements of 1/100,000 

 millimetre. (5) In the optical lever a beam of light falls 

 on a pivoted mirror ; if a body push the mirror at a point 

 very near the axis of the pivot the beam of light is deflected 

 by a large angle. By this means a movement of the body 

 by 1/400,000 millimetre may be detected. (6) The most 

 niodern and sensitive method, the electric micrometer, is 

 due to Dr. P. E. Shaw, who produced it in 1900, and 

 has improved it until he can now measure less than 

 1/2,000,000 millimetre. The nucleus of the apparatus is 

 shown in the figure. A fine screw m has a graduated head 

 N. The screw in rising pushes up the long arm of a 

 lever pivoted at B. The short arm of the lever falls, and 

 in so doing lets down the long arm of a second _ lever. 

 This process is carried on through six levers, which all 

 rest under their own weight on the blocks shown. The 

 last lever carries a measuring point p, just above which 

 is a measuring surface Q. If the Joint leverage of the 

 lever system be 2000/1, an upward movement of the screw 

 point M by i/iooo millimetre produces an upward move- 

 ment of P by 1/2,000,000 millimetre. 



As a simple example, suppose we wish to find the 

 thermal coefficient of expansion of the rod R, we proceed 

 thus : — Bring p and Q into contact. The screw M is 

 worked up, while a telephone (Tel.), in the electric circuit 

 shown, is on the observer's head. When p touches Q a 

 circuit is completed, and the telephone sounds. Read the 

 graduated disc n. Now lower the temperature of R by 

 any desired amount, taking care that little or no heat 

 reaches the pillars f' or any part but R. R contracts, and 

 by working screw M up the observer causes _P to touch g 

 again ; the telephone sounds, and N is again read. The 

 expansibility can thus be found, when we know the move- 

 ment of p and the change in temperarure. 



The screw, the levers, and the frame f' are all carried 

 by a massive girder i. The whole is surrounded by a 

 box thickly wrapped in felt to minimise temperature 

 changes, and is suspended by long rubber cords from the 

 ceiling, to insulate the measuring apparatus from vibra- 

 tion. 



The screw M is not touched by hand, but is worked 

 by a pulley cord of rubber which passes from a hand 

 pulley round pulley o. This is done to avoid the com- 

 paratively rough touch and tremor of the hand. There 

 are many precautions as to shape, size, cleanliness, and 

 other matters which must be observed. 



Based upon a paper by Dr. P. E. Sha 



ad before the Royal Society. 



