July 1, 1891. 



KNOWLEDGE 



133 



continued observations discovered many other radiant 

 points in the heavens from which the meteors which fall 

 at other times of the year appear to radiate. Mr. ]3enning 

 has most sj-steuiatically extended the work, until now 

 more than a thousand of suc-li radiant regions in the 

 heavens are knowu- 



Ojitinil I'rqjt'ctioii, it tiriitiae i>n tlw Uxr of tin LiiiitfDl. 15y 

 Lewis Wright (Longman, Green, & Co.) The lantern is 

 becoming more and more widely used by teachers and 

 lecturers, as it enables them to throw upon the screen 

 enlarged images of physical phenomena, and to perform 

 many experiments in the presence of large audiences which 

 formerly could only with difficulty be exhibited to two or 

 three at a time. But one still frequently sees demon- 

 strators who never half develop the powers of their 

 apparatus solely for want of duly considering its optical 

 properties. This does not apply only to itinerant lecturers, 

 but to polarization and other physical phenomena 

 projected with the most elaborate electric-light apparatus 

 at places which may be considered the very headquarters 

 of scientific exposition ; experiments are frequently shown 

 in a manner which gives inferior results to those which 

 may be obtained with only oxy-hydrogen illumination. 

 Such a book as Mr. Lewis Wright has given us will there- 

 fore be widely welcomed. It deals not only with the 

 optics of the lantern, but -with all sorts of lantern 

 accessories, screens, slides, the lime light, the preparation of 

 gases, &c. Several chapters are devoted to descriptions 

 of experiments suitable for projection on the screen, 

 many of which are ingeniously contrived. Mr. Lewis 

 Wright has had large experience in such work, and has 

 for years made it a special hobby. He was already 

 favourably known to us by his excellent text-book on "Light 

 and Experimental Optics," and the pi'esent work exhibits 

 equal resource and ingenuity. In projecting the apparatus 

 necessary for experiments, as well as pictures upon the 

 screen, it is not as easy as might at first sight be imagined 

 to arrange for an equal illumination of the field, and that 

 as many rays as possible may be made to fall upon the 

 slide or object, and also to pass through the projecting lens. 

 Mr. Wright has been especially happy in contriving means 

 to this end, suitable for very various classes of experiments, 

 and different degrees of magnification. He states that 

 the preparation of his book was suggested to him by Mr. 

 Herbert C. Newton, with whom he has spent much time 

 in contriving and testing scientific optical apparatus for 

 the use of colleges and public institutions, and the work 

 is in some senses a joint production, Mr. Newton's firm 

 having made this branch of optical manufacture a 

 speciality, have been enabled to otter Mr. Wright special 

 facilities for experiment, and the result is eminently 

 satisfactory. 



Hcttcvs. 



[Thu Eilitor does not hold himself responsible for the opinions or 

 statements of correspondents.] 



IIIK liKILLlANC'Y (IF DorHl.K STARS. 



To tin' Kditor af Knowledck. 



SiK, — Mr. Maunder falls into a mistake in supjiosing 

 that my formula in Tin- Ohsci-nitoni, Vol. X., gives the 

 " mass-brightness " or " candle-power per ton " of a binary 

 star. The proper expression would be " density-bright- 

 ness," not " mass-brightness," /.<•., the relative illumination 

 per unit of sm'face on the assumption that the stars were. 



in all cases, globes of equal density. This correction, 1 

 think, renders Mr. Maunders figures rather more favour- 

 able to his conclusion, but the error is worth correcting 

 as it occurs in Professor Young's well-known text book, 

 though it will no doubt be rectified in the next edition. 

 According to Mr. Maunders table, only one Slrian star has 

 a density-brightness lower than the average for Solar stars, 

 while in no case does the density-brightness of a Solar 

 star rise to the average for the Sirian class. I may notice 

 that my figures in The Ohxerratonj for the two stars X and 

 r Ophiuchi were vitiated by an error in the computation. 

 This has been corrected by Mr. Gore. 



Dublin, .June 6th. " W. H. S. Monck. 



EkI'LY of ilK. ilAUXDtll. 



I am obliged to Mr. Monck for his correction, which 1 

 had myself perceived the necessity for almost immediately 

 after I saw my article in type. It is curious how easily 

 one can make and pass over so obvious an error, which 

 looks " gross as a mountain " when the attention is called 

 to it. 



My mistake arose from a too natural interpretation of 

 Prof. Young's graphic but misleading expression of " candle- 

 power per ton," which caught my fancy and which I 

 followed up without examination. The process Mr. Monck 

 gave in I'lie Olmerratnri/, and which Mr. Gore and Prof. 

 Young have adopted from him, gives us the relative bright- 

 ness per unit of surface, assuming that the two binaries 

 have the same density. As Mr. Monck says, my mistake, 

 however inexcusable, does not vitiate my conclusions ; 

 rather the other way. Assuming that the illumination 

 per unit of surface is the same for all stars, and computing 

 the relative density, Mr. Monck's formula gives the 

 average Solar star as fourteen times as dense as the 

 average Sirian star. The net result, therefore, of such 

 materials for comparison between the two types as we at 

 present possess gives the preponderance to the Solar stars 

 in total radiation, in size, and in density ; and the more I 

 think over the facts we have before us — meagre enough, I 

 admit — the more I am inclined to consider the generally 

 received interpretation of stellar type as the result of an 

 over hasty generalization, and to consider that difference 

 of type of spectrum most generally indicates an actual 

 difference of constitution. 



E. W. Mainder. 



I'KRl'KTr \1. CAl.KNDAKS. 

 To the Eiliiiir III Knowledge. 



SiK, — Circvdar Calendars have been numerous. My 

 brother published one fifty years ago, so far as I remember, 

 similar to Mr. Prince's, but smaller. 1 bought one more 

 than twenty years ago in Paris, a very neat little thing, on 

 white metal, about the size of a two-franc piece, and some 

 ten years since one was described in the Kmilixh Mirlnuiii-, 

 consisting of three circles, one being for centuries. The 

 chief drawback to their use is their absence when wanted, 

 and the trouble of referring to the printed addendum to 

 find the Sunday Tjctter. 



The handiest mental rule 1 have come across, dispensing 

 with all reference whatever, is the following. By its use, 

 I think the week day for any date may be much more 

 readily found than by any circular calendar or similar 

 nieans. 



Rule : From half following Leap Year subtract years 

 past. In other words, from half the next greater multiple 

 of 4, take the remainder on dividing year by 4. Divide 

 by 7 ; remainder is date of first Sunday in March of given 



