386 



NATURE 



[August 19, 1897 



OUR ASTRONOMICAL COLUMN. 



Conditions for Best Telescopic Definition. — Dr. 

 T. J. J. See brings together a few facts and remarks regarding 

 the conditions essential to good seeing with large telescopes and 

 high magnifying powers {Astr. Nachr. , No. 3438). These are 

 based not only on his own experience under very favourable 

 circumstances during the past year, but many of the suggestions 

 developed are, as he says, the outcome of Mr. Douglass' work 

 on atmospheric currents and their relation to astronomical seeing. 

 At the Harvard station in Peru the seeing at three o'clock in the 

 morning was nearly always bad, caused, as was discovered, by 

 a current of cold air from the valley draining the great moun- 

 tains above and rushing down the adjacent gorge flowing over 

 the observatory, and completely ruining the seeing almost in- 

 stantaneously. Such currents as these must always be avoided 

 when fixing upon a position for an observatory, and this is one 

 of many causes which produce bad definition. The country in 

 which good conditions might be depended on should be free from 

 mountains and cyclonic causes which disturb the equilibrium of 

 the atmosphere. A high and dry table-land, distant from oceanic 

 influence, like the northern part of Arizona, presents conditions 

 which are almost ideal when snow is not present. Mountain 

 sites are always less satisfactory than broad table-lands, because 

 currents forced up from below are cooled by expansion due to 

 diminished pressure, and rapid changes are likely to take place 

 when the wind is strong. When covered with snow and over- 

 flown by currents of a different temperature, mountain sites are 

 wholly incapable of giving good definition. 



Astronomical Photography for Small and Large 

 Apertures. — In this column (April 8, vol. Iv. p. 544) we have 

 previously referred to the remarks which Dr. Isaac Roberts 

 published in Knowledge (vol. xx. p. 100) regarding the probable 

 limit in the length of the time of exposure for astronomical 

 photography. In these he showed that his experience led him 

 to conclude that in consequence of prolonged exposure to the 

 latent sky luminosity the film of the negative darkened on de- 

 velopment to a degree that would obscure faint nebulosity and 

 faint stars, and that longer exposures of the plates would not 

 reveal additional details of nebulosity, nor more faint star 

 images. 



Prof. F. L. O. Wadsworth is not, however, inclined to agree 

 with Dr. Roberts' statement in every particular, and contributes 

 to the Astronomischen Nachrichten (No. 3439) an article of 

 great interest, dealing with the question under discussion, in 

 which he states his reasons. This should be read by all who 

 employ the camera for astronomical photography, whether the 

 apertures they use be half an inch or twelve inches. We give 

 here the conclusions. 



The absolute intensity of the image of a celestial object, and 

 therefore the absolute photographic light action (product of in- 

 tensity by time), for a given time of exposure will vary (i) for 

 extended sources as the square of the angular aperture only ; 

 (2) for point sources as the product of the square of the angular 

 aperture times the square of the linear aperture. 



The contrast between the image of any celestial object (not 

 very near the horizon) and the general field depends upon (i) 

 the brightness of the sky at the time ; (2) the efficiency of the 

 image-forming lens as regards perfection of figure and curvature 

 of surfaces, &c.; and (3) upon the square of the linear aperture. 

 If the objectives are good, the sky effect (i) and (3) is the most 

 important. 



For faint extended objects, such as nebulae, irresolvable star 

 clouds, &c., in which we have to deal with the delineation of a 

 surface rather than with individual points, this contrast can only 

 be increased by decreasing the focal length. When the sky 

 effect (i) and (3) is predominant, it will vary inversely as the 

 square of the latter quantity. 



For point sources the contrast can only be increased by in- 

 creasing the angular aperture. Under the same conditions as 

 just mentioned in the last paragraph, it will vary directly as the 

 square of this quantity. 



It is the degree of contrast and not size of objective (except in 

 so far as this latter influences the contrast) that determines the 

 limiting magnitude of the faintest object that can be photo- 

 graphed. This limiting magnitude for stars depends, therefore, 

 only on the angular aperture, for nebulse on the focal length. 



The time of exposure also depends very largely on the con- 

 trast between image and field, and not on the absolute intensity 

 of the former. 



NO. 1 45 I, VOL. 56] 



As regards the influence of the character of the objective upon 

 the illumination of the field, the refractor seems to have a 

 decided advantage. The angular aperture of the latter should 

 not, however, be greater than i to 5. 



To photograph the very faintest stars (beyond 17th mag.) 

 a reflector of the largest possible angular aperture, i.e. I to 3 or 

 I to 2, if possible, is the only instrument that can be used. 



New Variable Stars. — Mr. Thomas D. Anderson com- 

 municates to the Astronomischen Nachrichten (No. 3440) the 

 discovery of a new variable star in the constellation of Hercules ; 

 its position for i855"o being R.A. i6h. 55 cm.. Declination 

 4-31° 26'. Mr. Anderson noted this star some time ago as 

 being a very faint star of the 9th magnitude, and of about the 

 same brightness as B.D. -t-3i°'2949. Several times in the 

 autumn of last year he was unable to see it with his 2*25 inch- 

 refractor, although he could always see a 9 "6 mag. star which is 

 not given in the B.D., but whose coordinates for 1855 are ap- 

 proximately i6h. 55"2m. and -)-3i°34'. This year, on the 

 22nd and 26th of last month, he has found the missing object, 

 and it was then brighter than the neighbouring star just men- 

 tioned. Its brightness was then estimated as being the same as 

 B.D. +3i"'2949 with a magnitude of 9-2. As a guide to tho.«e 

 who wish to observe this variable, Mr. Anderson says that 

 it lies further from +3i'''295i than from 2949, and also that 

 + 3i''"2945, 2946, 2949, the variable, and the 9*6 mag. 

 comparison star are nearly in a straight line. 



In the same number of the Astronomischen Nachrichten, Mr, 

 Stanley Williams gives a list of seven probably new variable 

 stars which he observed on his way to Australia and back. 

 These variables are, however, all of considerable southern 

 declination, but we may mention three of which the variability 

 " appears to be almost beyond doubt." 



1875- 



Star. R.A. Decl. 



h. m. „ , 



L 1713 Cceli ... 5 o-o .. -35 53 



S Antlise ... 10 23-8 ... -29 58 



L4959Crucis ... Ii 51-9 ... -55 37 



Planetary Notes. — At the oppositions of Jupiter in 1895- 

 96 and 1897, M. Quenisset made some interesting observations 

 of this planet, using a refractor of 16 centimetres aperture, which 

 will be found recorded in the Bulletin de la Society Astronomique 

 de France for the present month. Accompanying these are 

 some excellent drawings made by him during those periods of 

 observation. We notice that he has adopted the nomenclature 

 of Lord Ross, Knobel, and Campani for the different zones of 

 the planet by which the positions of special surface markings can 

 be easily located. Why should not all observers of Jupiter adopt 

 the same method, for would not comparisons of different observa- 

 tions be thus rendered more simple ? Dr. Fontsere's observa- 

 tions of Venus, which appear in the same number of the Bulletin, 

 were made in the first months of this year at the Barcelona 

 Observatory. The surface markings seem to have been 

 clearly seen and recorded, while projections on the terminator 

 and limb were very commonly visible. This observer deduces 

 a long period of rotation for this planet. The observations made 

 by Dr. Peyra during the 1896-97 opposition of Mars appear irv 

 the Memorie delta Society degli Spettroscopisti Italiani(yo\. xxvi. 

 4^). These were made with a 24-centimetre Merz refractor, 

 and are well worth comparing with those of other observers 

 made about the same time. The drawings accompanying the 

 observations are on rather a small scale, and show only the 

 more prominent markings and canals. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 



Dr. Rodet, the well-known bacteriologist of Lyons, has 

 been appointed Professor of Bacteriology in the University of 

 Lyons. 



Mr. Stanley Dunkerley, of the Department of Applied 

 Mechanics, Cambridge, has been appointed Professor of Applied 

 Mechanics at the Royal Naval College, Greenwich, in succession 

 to Prof. J. H. Cotterill, F.R.S., who is about to retire after 

 over twenty-four years' service. 



The candidates successful in this year's competition for the 

 Whitworth scholarships and exhibitions are as follows :— ( i ) 

 Scholarships of 150/. (tenable for three years) : George M. 



