'ULY 5, 1906] 



NA TV RE 



231 



other drops arc found to be 2, 3, 4, 6, &c., times thai 

 wiifjht. Generally speaking, the absolute size of the drops 

 exhibits very small differences; the smallest wei}<h about 

 oil mij., and the weights of the others are multiples of that 

 value, :i> explained above. In a lasting downpour the largest 

 droj)-. woigh about i mg. With respect to the velocity at 

 which the drops fall, the rate is not at all proportional 

 to the weight; those of on mg. to i mg. fall at the rate 

 of 2-7 metres to 44 metres per second, while those of 

 exceptional weight, say 65 mg., only fall at the rate of 

 about S metres per second. These rates only hold good 

 durin;; c.ilm air; in an ascending current of 2-7 metres 

 per si-cond the smallest drops would remain suspended. 

 The latter part of the article deals with Mr. Wilson's 

 experiments on the ionisation of the atmosphere as the 

 prcibable prime cause of the formation of rain. A note on 

 the si/e of raindrops will also be found in vol. xviii., 

 p. J42, of the Quarterly Journal of (he Kuy.A Metciir- 

 ological Society. 



.\n interesting contribution to the study of the nature 

 of solution and of osmotic pressure is contained in a paper 

 by Mr. C. S. Hudson in the Physical Rcviciv (vol. xxii., 

 No. 5). The conception of Prof. Hulelt that the low 

 vapour-pressure of solutions indicates that these solutions 

 are under a negative pressure is extended to explain the 

 depression of the freezing point of water by the addition 

 of a dis.solved substance. It would appear at first sight 

 that a negative pressure would occasion a rise in the 

 freezing point, because an increase of pressure causes a 

 lowering of the melting point of ice ; but this reasoning is 

 not correct, because the ice which freezes from a solution 

 is under atmospheric pressure, not negative pressure, and 

 only the solution may be regarded as being subjected to 

 the negative pressure. By using Prof. Poynting's calcula- 

 tion of the change of freezing point caused by an increase 

 in the pressure on the ice alone, it is shown that the 

 molecular depression of the freezing point of water caused 

 by the addition of a dissolved substance is exactly equal 

 to that corresponding with a negative pressure exerted on 

 the solvent, and equal in magnitude to the osmotic 

 pressure. The osmotic pressure thus corresponds with a 

 positive tension exerted on the liquid by the dissolved solid. 

 A general thermodynamic investigation of the process of 

 freezing is also contained In the paper. 



A FOURTH edition of Mr. C. F. Townsend's " Chemistry 

 for Photographers " has been published bv Messrs. Daw- 

 barn and Ward, Ltd. 



OUR ASTRONOMICAL COLUMN. 



Searcii-ephemeris for Finlay's Comet. — In No. 4100 of 

 the Astrorwmische Nachrichten M. L. Schulhof publishes 

 an approximate dally ephemeris for the coming apparition 

 of Finlay's comet. The time of perihelion passage is taken 

 as .September 8, and the ephemeris covers the period 

 June iS to .August i ; two contracted ephemerides are also 

 given for T = September 40 and T = September 120. 

 Owing to its expected close approach to the earth, the 

 comet should be in an excellent position for observing later 

 in the year. 



The perturbations since the last apparition of this comet 

 have not been taken into account in the present ephemeris, 

 but a more accurate ephemeris is promised In an early 

 publication. According to that now published, the comet 

 is at present (July 5) apparently near to S Aquarii, which 

 rises about 11 p.m., but by the end of the month it will 

 have passed into Cetus, and will be about half-way between 

 e and Mira Ceti. 



NO. 1914 VOL. 74] 



Stereoscopic Measurement of Proper Motions. — A de- 

 tailed description of the stereo-comparator method of 

 determining the proper motions of stars is given by Prof. 

 Max Wolf in No. 4101 of the Asironomische Nachrichten, 

 where he also gives and describes the first results obtained 

 by the method. A pair of photographs, arranged for the 

 stereoscope, which accompany the paper show the effect 

 of proper motion beautifully, for a star which has moved 

 iq seconds of arc in fourteen years appears to be con- 

 siderably behind the general plane of the surrounding stars. 

 With an especially constructed micrometer, the observer 

 is able to determine the amount of the proper motion in 

 right ascension and declination. 



Prof. Wolf shows in a table the values measured and 

 the results obtained for ten stars of about the tenth 

 magnitude, and also for two other stars, Nos. 75 and 74 

 in Prof. Kobold's list. Comparing the meridian-observation 

 results for the latter with the stereo-comparator values, he 

 shows the trustworthiness of the now method thus : — 



Meridian ciicle ... 2-23 

 Stereo-comparator 2-21 



This is very satisfactory, especially when one remembers 

 that on the scale of Prof. Wolf's plates i second of arc 

 is represented by only 0004 mm. 



Radiative Power of the Sun's Disc— In No. 4, vol. 

 xxiii., of the Astrophysical Journal, Prof. Julius describes 

 a new method for determining the radiative power of the 

 different parts of the solar disc. Briefly, the method con- 

 sists in recording the intensity of the solar radiation at 

 definite intervals during the progress of a total solar eclipse. 

 Then every increment (either positive or negative) of the 

 intensity is solely due to the radiation coming from that 

 strip of the disc through which the limb of the moon has 

 appeared to move during the corresponding interval. As 

 the geometrical form of each of the strips is easily deter- 

 mined the amount of each of the concentric zones (into 

 which the disc is previously divided) contained in any orie 

 strip may be found. The total radiation from each zone is 

 then determined bv a suitable mathematical solution. This 

 method was tried' at Burgos during the last eclipse, and, 

 despite the unfavourable meteorological conditions, the 

 results lead Prof. Julius to hope that under suitable con- 

 ditions it may be found very satisfactory. 



New Forms of Astrographic Object Glasses.— In No. 

 4100 of the Astronomische Nachrichten M. Emil Schaer 

 ifescrlbes a novel method of constructing a short-focus 

 astrographic objective. Two lenses of the usual crown and 

 dense flint glasses are employed ; the crown is placed in 

 front of the flint, and the back surface of the latter is 

 silvered, so that the photographic plate has to be placed in 

 front of the combination at the combined focal distance. 

 M. Schaer has tried this method with two discs of 280 mm. 

 (about II inches) aperture, made for him by M. Mantois, 

 and, by suitablv figuring the back surface of the flint 

 before silvering 'it, has obtained an objective of 89 cm. 

 focal length which is practically free from aberration 

 effects, and has a large light-gathering power. To obviate 

 unnecessary reflections, the two lenses were stuck together. 



Another 'innovation in the construction of objectives is 

 announced in No. 597 of Science, where it is stated that 

 a Hungarian chemis't, after many years' experimental work 

 has succeeded in manufacturing perfectly satisfactory fluid 

 lenses. The fluid is hermetically sealed between two hard 

 glass surfaces, similar to watch crystals, the glass being 

 chosen so that the combination is achromatic. 



The inventor claims that an objective, equal in practice 

 to any yet made, of 150 metres aperture can be made in 

 a few weeks at a cost of 2000 or 3000 marks {i.e. about 

 150!.). 



These lenses are already being manufactured, and are 

 giving satisfactory results, in Austria, and patents are 

 ijeing taken out in other countries where they are soon to 

 be introduced. 



