470 



KNOWLEDGE. 



December, 1910. 



16 was leading uj) to this conclusion, at the ven- end 

 I find, on the contrar\-. ten Commissioners agreeing 

 that some unfortunate invertebrate ought to be 

 named Clujos clmos .' I can onlv hope that at some 

 future revision it ma\' be called Toltii vahohii. 



One opinion is conspicuous by its absence, and 

 will, it appears, not put in an appearance in the 

 report of the Congress held at Graz last August. It 

 concerns the question of genders in Zoology, uhich 

 was submitted to the Commissioners years ago h\' a 

 learned Society and recently recalled to their attention 

 bv copies of an article from these columns. This 

 question is therefore left to be determined by the 

 usage of authors, and I can onl\- regret that, while 

 modestly waiting forthe opinion of the Commissioners. 

 I have neglected favourable opportunities of giN'ing 

 practical effect to my own. To show in a striking 

 light the needless perplexity produced by forcing 

 Latin concords upon natural science, let me offer a 

 few more examples from authorities, most of them 

 of high repute. Latreille is responsible for Tenucs 

 capciisis and Tcniics fiisciini: Milne-Edwards for 

 Hippolytc vcntricosiis : Kro\"er for Tticca imprcssiis: 

 Brad\- iov Poiitella necitrifer: G'leshrechxiov La bid oc era 

 aciifiinr. but Giesbrecht and Schmeil for Lahidoccra 



acuta: Ortmann for Sesanna cafeiiafa: Stebbing for 

 Scsarma catenatum. How many naturalists could, 

 without an inordinate amount of trouble, discriminate 

 between the right and the wrong in these genders, or 

 e.xplain the origin of an\- errors committed? 



One there was among the Commissioners who, 

 with more ease than most men, could have accepted 

 this challenge. Dr. von Maehrenthal was present at 

 the recent Congress, but is, alas, no longer among 

 us. As assistant editor of " Das Tierreich '' he was 

 Professor F. E. Schulze"s invaluable comrade, loyal, 

 indefatigable, self-effacing. His loss may well be 

 called irreparable, unless it may induce the Berlin 

 Academy to take into alliance kindred institutions in 

 other nations for carrying out the gigantic task they 

 ha\'e in hand. To give an idea of the distance 

 between us and our chief luminary Sir Robert Ball 

 imagined an infant with an arm capable of putting 

 its fingers on the sun. reckoning that the sensation of 

 any consequent blister would not reach the brain till 

 the child had become an old man. Similarh'. in the 

 task of describing all the animals of the world, how- 

 ever we name them, there is a risk that, without a 

 combined effort, the beginning will be long out of 

 date before the end is arrived at. 



THE SPFXTROHELIOGRAPH. 



By FR.\NK C. DENNETT. 



The most powerful instrument in the hands of the solar 

 physicist is the spectroheliograph. The focal image of the 

 sun, formed by an object-glass of very long focus, is made to 

 fall on the slit plate of a spectroscope, the slit of which is 

 greater in length than the diameter of the image falling upon 

 it. As the object-glass has a long focus, that of the collimating 

 lens must be proportionally long. The diagram (Figure 1) 

 shows the optical contents of the prism 

 box of the Mount Wilson " Rumford " 



spectroheliograph. C is the tube of the 



collimator ; D was formerly a plane 

 mirror, but is now replaced by a ^ 



diffraction grating ruled with twenty \^ 



thousand lines to the inch ; P P two 



light flint glass prisms 60" angle. The 



spectrum, instead of being examined 

 with a telescope, is caused to fall 

 upon the sensitised plate which re- 

 places the eyepiece of the observing 

 telescope. Before reaching the plate, 

 however, it is intercepted by a second 

 slit, which cuts off all the light save 

 that passing through the tiny portion 

 of the spectrum required — say the 

 K line of Calcium, or the C line of 

 Hydrogen. The image of the sun 

 travels slowly across the first slit, 

 and the photographic plate is caused 

 to travel behind the second slit, by 

 clockwork, at a corresponding rate. * 

 Two exposures are made when 

 prominences and disc are to be 

 photographed. A slower exposure 

 for the prominences, the disc being hidden by a screen, and a 

 second rapid exposure, when the screen is removed, for the 

 disc. The total deflection of the ray is 180°, the prisms doing 

 the double work of increasing the dispersion, and getting rid 

 of the small amount of scattered light caused by the diffraction 

 plate. Hydrogen prominences and flocculi are best studied in 



FlGL'Ki: 1. 



photographs taken in the red light of C. The greater the 

 density of the gas or vapour causing a line, the greater also is 

 the width of the line. The closer a vapour is to the sun, the 

 greater is its density also. Hence it is found that by taking 

 photographs with the light of the middle of a line and com- 

 paring it with that of the edges of the line a different image 

 is obtained. The photograph obtained with the centre of the 

 line is largely that of the upper clouds, 

 whilst that with the edge of the line is a 

 picture of the lower, denser masses. The 

 dift'erence in the case of the C line of 

 Hydrogen is well shown in the plate 

 in '■ Knowledge," facing p. 4 of the 

 present volume. 



The spectroheliogram now repro- 

 duced, kindly sent by Professor 

 George E. Hale, from Mount Wilson, 

 was taken July 17th, 1907, in the light 

 of calcium. The prominences around 

 the southern hemisphere are very fine. 

 The tall tree form on the south-western 

 limb has an altitude of eighty-seven 

 thousand miles. The beautiful double 

 arch south east extends over a 

 distance of two hundred and thirty- 

 four thousand miles, the taller arch 

 being fifty-five thousand miles high, 

 whilst the cloudiness over the southern 

 arch reaches over eighty-two thousand 

 miles in height. The great calcium 

 clouds, or flocculi, over the faculae of 

 the great group near the centre cover 

 an area over one hundred and thirty- 

 seven thousand miles east to west. The less dense mass 

 about north, and preceding the more southern great spot, 

 reaches two hundred and thirteen thousand miles in the same 

 direction. This last group presents one peculiar feature — 

 the chain of almost equi-distant openings extending through 

 nearly its whole length. 



