August, 1905.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



205 



using a Wallace screen, he secured a good picture of the Mare 

 Acidalium. Encouraging as the result was, there were no si,!,'ns 

 of any canal markings. The two chief difficulties were the 

 variation of the atmospheric tremors, and the insufficient 

 sensitiveness of the photographic plates. The endeavours to 

 get rid of these errors resulted in the ordering of a film camera 

 wherewith a succession of pictures could be rapidly taken 

 behind a Wallace screen ; and with this arrangement Mr. 

 Lampland has got his interesting results. A most important 

 item, however, has been the suitable cutting down of the 

 aperture of the photographic telescope to suit the particular 

 state of atmosphere at the time of observation. 



From the many plates secured, the one taken on May 11 was 

 selected for the reproduction sent with the circular. Side by 

 side with the print is placed a photograph of a drawing by 

 Professor Lowell, made before the camera was placed in posi- 

 tion, and this serves the double purpose of showing the con- 

 firmation by the photograph of the objectivity of the visual 

 observation, and at the same time of serving as a chart to it. 



The print is enlarged i'8 times from the original negative ; 

 and not only are the canals easily discernible, but it is evident 

 that they are continuous lines, and not syntheses of other mark- 

 ings, as has been suggested by various writers. 



An additional note by Mr. Lampland states that the photo- 

 graphs were obtained with the 24-inch Clark refractor of 

 386 inches focal length. The camera carries a negative enlarg- 

 ing lens, the equivalent focal length of the combination being 

 148 ft. The camera carries a plate holder for 3^ x 4i plates, 

 movable perpendicularly to the optical a.xis, thus permitting a 

 dozen or more exposures on the planet, for the focal length 

 given, to be made on the same plate. 



A colour screen is placed immediately in front of the plate, 

 separated by a small space to minimise the effect of small 

 particles of dust or other extraneous matter. Cramer's 

 isochromatic plates were used, this make being chosen on 

 account of the fact that one of the maxima of the curve of 

 sensitiveness of the plate coincides almost exactly with the 

 vertex of the colour curve of the large objective. The best 

 results have been obtained with the 24-inch stopped down to 

 9 or 12 inches, and the exposures were usually about eight 

 seconds each with the 12-inch aperture. 



Spectroscopic Observations of Mercury 

 during Solar Eclipse. 



Dr. G. Johnstone Stoney draws attention to the opportunity 

 which will be aflbrded during the approaching total solar 

 eclipse on August 30 of obtaining important observations of 

 the planet Mercury. At that time Mercury will be very close 

 to the line joining the Earth and the Sun ; its centre is 2^ 54' 

 South, and 2° 54' preceding the Sun, so that the distance is 

 only about 4° 6', and the illuminated portion of the planet's 

 disc will be seen as a very fine crescent. If the planet 

 possesses any appreciable atmosphere, the horns of this 

 crescent will be prolonged by the effects of atmospheric 

 refraction, and micrometric measurements of the degree of 

 such elongation would furnith material for calculating the 

 extent of the planetary atmosphere. For such an observation 

 a telescope magnifying about 200 diameters would be desir- 

 able. Further important and interesting determinations may 

 be made by treating the thin crescent as a slit, and viewing it 

 through a spectroscope, as then the exact constitution of the 

 atmosphere surrounding the planet might be ascertained. 



Photographic Studies of the Planet 

 Mars. 



Quite recently a series of successful photographs of the 

 planet Mars have been obtained under the direction of Pro- 

 fessor W. H. Pickering at the Harvard College Observatory. 

 In the spring of the present year the 11 -inch Draper telescope 

 was fitted with an enlarging lens, and it was found possible to 

 obtain original negatives showing the disc of the planet on a 

 scale of about 2"' 5 to the milUmeter. 



The first photograph was obtained on March 31, and others 

 were secured on April i, 2, 8, 15, 16, iS, 23, 25, 27, 30. The 

 first photograph showed clouds at both the limb and ter- 

 minator, but no definite evidence of actual polar caps was 

 visible on the photographs until April 23, when a large light 

 area was clearly visible at the south pole. It did not appear 

 dark enough for snow, but presented more the appearance of 



an extensive cloudy region. It remained visible on all the 



photographs since that date, although its intensity and size 

 diminished somewhat. A minute light area appeared near the 

 north pole of the planet on April 15, but was seen only with 

 difficulty. 



On the night of April 30, visual observations were made 

 with the 24-inch reflector. The southern polar cap was 

 clearly visible, extending far to the north in longitude 340', but 

 its intensity was only slight, little exceeding that of the limb in 

 other regions. It is considered probable that when the Mar- 

 tian clouds disperse snow may be found lying in their places. 



Considering briefly the aspect of the planet at these times, 

 we notice that the heliocentric co-longitudes on April 15 and 23 

 were 216° and 220° respectively. These positions would cor- 

 respond on the earth to August 3 and 7, or to near the end of 

 the winter of the southern hemisphere. Snow seldom comes 

 earlier on Mars. It will be very interesting to observe if the 

 brown tint described by Lowell as characteristic of the Mare 

 Erythrsium will become changed to its normal colour. This 

 change of colour with the seasons may yet afford the best 

 proof of the existence of vegetation upon the planet Mars. 



Ephemeris for Observations of Comet 

 1904 I. 



This Comet will be somewhat near the Sun, and should be 

 looked for a little before sunrise, in the constellation Lynx, 

 above Cancer. 



,J^^^^^ 



CHEMICAL. 



By C. A. Mitchell, B.A. (Oxon.), F.I.C. 



The Flashing of Arsenic Crystals. 



Arsenious oxide, the ordinary ichite arsenic of commerce, 

 forms two distinct modifications, differing from each other in 

 specific gravity, melting point, and other physical properties. 

 The vitreous modification is semi-transparent, but on exposure 

 to the air gradually becomes opaque and of a yellowish tinge 

 as it changes into the other vatiety. As far back as 1S35 it 

 was found by Rose that when the vitreous modification was 

 dissolved in boiling hydrochloric acid the excess of uncombined 

 arsenic separated out in minute crystalline octahedra from the 

 solution on cooling, and that on shaking the contents of the 

 flask in the dark a succession of brilliant flashes was emitted. 

 The generally accepted explanation of this very beautiful 

 phenomenon is that at the moment when the crystals separate 

 the vitreous modification is suddenly transformed into the 

 crystalline variety, the change being accompanied by a 

 liberation of energy expressed in the form of light. It has 

 recently been shown, however, by M. D. Gernez {Coniptes 

 Renclns, May, 1905), that this explanation is incorrect. He 

 finds that if the flask be kept absolutely still the formation of 



