December 30, 1909] 



NA TURE 



261 



down to I inch. It is extremely quick for comet work. 

 Its focus sliould be carefully determined by star trails. 



In comet work it is important in all these lenses that 

 the camera should be so adjusted on the mounting with 

 respect to the guiding telescope that the head of the comet 

 can be displaced to one side of the field to secure a greater 

 extension of the tail. Two of the small lenses may be so 

 arranged by a mutual adjustment as to cover the full length 

 o! the tail, even though it should be fifty degrees long or 

 more. Although it would thus be in two sections — the 

 head and part of the tail on one plate, and the rest of the 

 tail on the other — there would be no serious objection if 

 the whole tail could thus be secured. The large reflectors 

 will be of the utmost importance in deaUng with the detail 

 and structure of the head and envelopes, as has been 

 recently shown at Greenwich. 



Until something further is known of the spectrum of 

 the comet, it would be unwise to attempt to give any 

 specific directions as to the duration of exposure required 

 with any telescope. Daniel's comet of 1907, and More- 

 house's of 1908, were very different in respect to their 

 photographic activity. The latter was relatively many times 

 more actinic in its light, and hence required much shorter 

 exposures to show the same strength of tail. This in- 

 formation must ^ome from actual experience with the 

 comet. It would seem, however, that the circumstances of 

 the comet's visibility when brightest will make short ex- 

 posures necessary. 



The committee will be pleased to receive from every 

 astronomer who maj' cooperate in the matter copies (glass 

 positives) of his negatives of Halley's comet, and it will 

 undertake the comparison and discussion of the material 

 thus collected. 



Spectroscopic Observations. 



For spectroscopic observations of the comet the com- 

 mittee makes the following suggestions, formulated by 

 Prof. Frost. While it may be possible to make visual 

 observations of the comet's spectrum with ocular spectro- 

 scopes attached to large telescopes, it is likely that most 

 of the photographic records of the spectrum will be obtained 

 by the objective prism or the slit spectrograph, and refer- 

 ence will be made in what follows to the use of these two 

 t3'pes of instrument. 



These methods of observation are mutually comple- 

 mentary; for the accurate measurement of wave-length, 

 effect of motion in the line of sight, and analysis of struc- 

 ture of lines or bands (if sufficiently sharp), the slit spectro- 

 graph has all the advantages, but for study of distribu- 

 tion of elements in different parts of the comet, and for 

 reaching faint details, the prismatic camera, or objective 

 prism, with its much greater light-power, is essential. 

 The prismatic camera may be employed, with a fair possi- 

 bility of success, when the comet's brightness is equivalent 

 to that of a ninth- or tenth-magnitude star; the slit spectro- 

 graph cannot be hopefully applied before the comet is two 

 or three magnitudes brighter. The size and kind of tele- 

 scope employed, of course, make such statements relative 

 rather than absolute, and uncertain at best. Too much 

 here depends upon the comet ; if its light is chiefly reflected 

 rather than intrinsic, and the continuous spectrum is pre- 

 dominant, then the comet will have to be much brighter 

 for satisfactorv spectroscopic analysis than if the light is 

 largely intrinsic and concontr.nted at half a dozen points 

 ii the spectrum. Comets showing sudden and marked 

 fluctuation in size or brightness are likely, to exhibit changes 

 in the bright band spectrum. 



(t) Prismatic camera or objective prism. 



The camera should be a doublet of large angular aper- 

 ture, 1/4 or 1/5. Useful observations could be secured if 

 the linear aperture is as small as 4 or 5 inches (10 or 

 12 cm.). The objective prism should be of small angle, 

 perhaps 10° or 15° ; if an additional prism is available for 

 the period of the comet's greatest brightness, its angle 

 should be about three times that of the smaller prism. 

 If the doublet is of comparatively short focus, as is likely 

 to be the case, it will be found to be quite sensitive to 

 focus, and separate exposures will be needed for the blue- 

 violet region and the yellow-green region. Optical parts 

 transparent to ultra-violet would be useful, as there may 

 be some important bands of shorter wave-length than 

 A. 3SS3. 



NO. 2096, VOL. 82] 



It is very desirable to photograph the spectrum of a star- 

 before or after the comet, placing the star at such a point 

 on the reticle of the guiding telescope that the stellar Unes 

 may serve for comparison. 



Prof. Pickering suggests that an interesting observation 

 would be to photograph the spectrum of a star when seen 

 through the bright portion of the tail, to see if dark, 

 absorption lines could be detected. 



(2) Slit spectrograph. 



.«\ small spectrograph will be a very useful attachment 

 to a photographic doublet as described above. It need not 

 be complicated, and its dispersion may be small. Apparatus 

 I for producing a comparison spectrum is not essential, for 

 a neighbouring star of the first type may be brought upon 

 the slit, and its spectrum impressed above and below that 

 of the comet. With one thread of the guiding telescope, 

 or finder, movable, the star can be placed so that its spec- 

 trum will fall on the slit as desired. The identity of the 

 cometary band can be thus established better than with the- 

 objective prism. After spectrograms of the comet have 

 thus been obtained, it will also be desirable to obtain plates 

 with the slit as narrow as feasible, in order to detect 

 duplicity or complexity of the lines or bands. 



Observations with powerful stellar spectrographs of the 

 types in use for determining radial velocities will doubtless 

 be made as soon as the comet's brightness permits, but 

 this is likely to be disappointingly late on account of the 

 heavy loss of light in such instruments. The fixed equip- 

 ment of these instruments will determine their operation by 

 their regular observers. 



Photometric and Polariscopic Observations. 



Photometric and polariscopic observations of the comet 

 should certainly be made, although they will doubtless 

 occupy a position of subordinate importance. The sugges- 

 tions of the committee in this respect are formulated by 

 Prof. Pickering, as follows : — 



A great variety of methods may be employed for measur- 

 ing the light or amount of polarisation of the comet. It 

 is suggested that astronomers undertaking this probleni 

 should correspond with the chairman of the committee, in. 

 order that uniform methods may be employed throughout 

 by different observers. The plans proposed below may 

 require modification, according to the instruments avail- 

 able. 



A direct estimate, by Argelander's method, of the entire 

 light of the comet, as seen by the naked eye, or in the 

 smallest telescope with which It is visible, may have a 

 certain value to observers in the future, although large 

 systematic errors are to be expected in such estimates. 



It is doubtful if photometric measures of the nucleus of 

 the comet will have much value, as the results will prob- 

 ably be greatly affected by the coma, and will differ with 

 different photometers and' telescopes. If the nucleus be 

 distinctly stellar it may be compared directly with an 

 adjacent star, by means of a double-Image photometer. 

 The effect of background would thus be eliminated. Direct 

 measures with a Zollner photometer, or similar instrument, 

 would probably have but little value, owing to the effect 

 of the coma. Any series by the same observer with the 

 same instrument would be valuable by itself, and the 

 observations by different persons and different instruments 

 might be subsequently adjusted for systematic differences. 



The measures described In Harvard Circular 68 showed 

 that the absorption of light by the tail of comet 1902 & 

 was certainly less than a tenth of a magnitude. Similar 

 measures should be made of Halley's comet. .\ double- 

 image photometer is indispensable for these measures also, 

 to eliminate the effect of background. 



The light of different portions of the tail of the comet 

 mav best be measured by the following method. Take 

 two photog'-aphs at the same time with similar instruments, 

 using the" same kind of plate and developer, and giving 

 equal e.xposures, taking one in focus and the other out of 

 focus, so that the images of the stars shall appear^ as 

 circles two or three millimetres In diameter. Make similar 

 enlargements of the two plates, interposing screens of per- 

 forated brass. Measures of the opacity of the resulting 

 circular images of different portions of the comet on one 

 plate may be compared with the images of stars the 

 magnitudes of which are known as photographed on the 



