138 ASTRONOMY FOR 1889, 1890. 



method, and doubtless far surpassing the preseut methods in ease of 

 measurement and output of work. 



The Henry Brothers are reported to have made a decided advance 

 in lunar photography in the plates taken with the equatorial of 0'".32 

 (12.C inches) api-rtiire intended for the chart work. The improvement is 

 attributed especially to the process of enlargement employed, which 

 makes the diameter of the moon about 1"> (39 inches). This photographic 

 work is to be continued with the great equatorial coude, wiiich is soou 

 to be mounted and provided with a photographic objective. 



Mr. Tioberts has devised a machine, which he calls a "paiitograver," 

 for measuring the magnitudes of the stars depicted upon the photo- 

 graphic plates and transferring them to metallic plates for printing. 



COMETS. 



The origin of comets. — Dr. Bredichin, the i)resent director of the Pul- 

 kowa Observatory, who has devoted much tiuie to the study of comet- 

 ary i)henomena, has expressed theopinion that periodic cometsowe their 

 origin to the segmentation of ordinary parabolic comets, having been 

 thrown off from the latter by an eruption such as it is generally sup- 

 posed we have witnessed in the great comet of 1882, and earlier in 

 Biela's comet. Dr. Kreutz's monograph on this great September comet 

 of 1882 forms one of the most important of recent contributions to com- 

 etary literature. The formidable obstacles to an accurate determination 

 of its orbit presented by the disintegration of the nucleus into several 

 points of condensation seem to have been most skillfully surmounted 

 by the computer. His linal value for the period of revolution is 772.2 

 years. 



Dr. Holetschek claims that the systematic grouping of cometary peri- 

 helia in certain directions (270° and 90° of heliocentric longitude) has 

 no connection with the general motion of the solar system in si)ace, 

 but is due to the position of the earth at the time that such discoveries 

 are most readily made. 



An important paper on the capture theory of comets will be found 

 in the Bulletin Astronomique for June, 1889, and in the same journal for 

 December, 1890, M. Tisserand has a further contribution to the same 

 subject. 



The Observatory for August, 1889, has a useful table of the approxi- 

 mate positions at the time of discovery of all comets seen since 1869, 

 with brief notes on the physical appearance of each. Mr. Denning, who 

 has compiled this table, proposes to supplement it by one with similar 

 data for the comets from 1840 to 1808. 



Brorsen^s comet. — A careful sear^^h for Brorsen's comet, which passed 

 perihelion in 1890, was made by Brooks and Swift, but without effect. 

 This comet was discovered in 184G, and was last seen in 1879; it could 

 not be seen at the return in 1884. Tempel's second comet, and Bar- 

 nard's comet 1884 II, were also expected to return to perihelion in 



