JONE, 1902.] 



K NOWLEDGE 



131 



exactly the apparent size of Venus in transit — would not 

 in all cases correspond jirecisely to the limit for the visi- 

 bility of spots. Specially dark and compact spots have 

 been distinctly seen even wlu'u only of one-half this size; 

 faint or scattered ijroups niiifht escape notice cvc-n if a 

 little lar^^jer; but jfeneral expericnre has shown that the 

 limit selectcil corresponds very nearly to the limit of 

 visibility without optical assistance, differing from it in 

 under-stating, not oveF-stating, the numlier of groups 

 which could l>e seen. 



Of course, there is no possibility now for the " Astro- 

 nomer without a Telesco]ie " to improve on the information 

 which the telescope lan •,'ive us as to solar physics, but the 

 lesson which the foregoing table has to teach is a mo<t 

 important one. Observations, as difficult and as apparently 

 hojieless as observations of the solar surface would have 

 seeraetl to be in the Middle Ages, may, if Cxuried out 

 l>atiently and systematically, bear as rich fruit as solar 

 observations could have done even then. Even after the 

 invention of the telescope, it was not the optical power of 

 his instrument, but the perseverance with which he 

 worked at a single object which revealed to Schwabe the 

 secret of the solar period. He had no dream of the dis- 

 covery before him when he set out upon his researches. 

 His own expression was that he "sot out like Saul, looking 

 for his father's asses, and fo\uid a kingdom." There are 

 kingdoms yet to be won, even in those fields of astronomy 

 which the telescope cannot touch. In particular, changes 

 which are periodic in character will reveal the fact and 

 circumstances of that periodicity to observations carried on 

 patiently and continuously, even when the amount of such 

 changes at their maximum only just come within the 

 utmost limits of the power of the instruments used in the 

 work'. 



THE BELT AND SWORD OF ORION. 



By Alex.^xdku Smitu. 



The ])hotograph of which the accompanying plate is a 

 reproduction — enlarged to about 2 diameters — was taken 

 on February 2 last with an exposure of 2 hours 40 minutes, 

 the lens used being a o|-in. Voigtliinder doublet of 

 22 inches focus. The camera was attached to the tube of 

 a I2j-iu. reflector, which was utilised as a guiding instru- 

 ment, while the driving power was furnished by a three- 

 pendulum turret clock, whose motion is so regular and 

 certain that very delicate photographic work can be under- 

 taken without risk of failure resulting from any unsteadi- 

 ness of the driving mechanism. 



The development of the plate was prolonged as far as 

 possible with the view of bringing out faint stars, and, 

 although this treatment has obscured all detail in the 

 brighter portion of the Great Nebula, there is little ditfi- 

 culty in tracing on the original negative almost the whole 

 of the fainter outlying vnsps of nebulous matter shown on 

 photographs, which have been secured, not only with 

 exposures of longer duration, but with much larger aper- 

 tures. A comparison of the photographs of this object, 

 which have been published from time to time, shows that 

 the whole region of the nebula exhibits more or less of 

 structure, and the result of a number of experiments I 

 have carried out recently would seem to indicate that 

 satisfactory impressions of such details can only be 

 obtained on a single exposure by departing from the 

 methods of development usually adopted. 



Another interesting nebidous region in the constellation 

 of Orion is that surrounding the star Zeta. Following 

 this star is H. V. 28, which is made up of several well- 

 defined patches separated by dark rifts in which there is 



very little trace of nebulous material. Stretching south- 

 wards from Zeta for about a degree is a straight band 

 of nebulous m.atter, which, although exceedingly difiicult 

 to detect visually, can hardly fail to Ix" noted on nega- 

 tives obtained with fairly long exixisures. In the /'. 

 edge, which is well defined, is u very prominent gap 

 somewhat rescMubling the "fish-mouth" in (he 6re;it 

 Nebula. In the same region there are several ui'bulous 

 stars. 23 min. south of H. V. 28 is H. IV. 24, and alioul 

 a similar distance /. is another and smaller nebuNnis star. 

 About 25 rain, north of Zeta is a third object of the same 

 class. These can easily be traced on the negative, while a 

 fourth, about 11 min. preceding the last named, is shown 

 on a recent j>hotograph taken by Mr. Henry Ellis with a 

 20i-iH. mirror. Only one of these objects is recorded in 

 the N.G.C., and the region would appear to bo worthy of 

 further investigation by those having large apertures at 

 their disposal. 



Hettfrs. 



The Kilitors do not hohl tliemselves responsible for tlie opinions 

 or statements of corres|iondents. ^ 



FAHRENHEIT'S THERMOMETER. 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — The description which I gave of Fahrenheit's 

 thermometer was merely the result of my enijuiries, and 

 was intentionally brief, as I feared to overload your 

 valuable columns. It has called forth the criticism of 

 Mr. Cleveland Abbe, of the Department of Agriculture at 

 Washington, who throws doubt on the statement that 

 Fahrenheit was indebted to Newton for his scale. It is 

 true that neither in Fahrenheit's biography, or indeed in 

 any work of the time, is there any distinct account of the 

 rationale of his scale ; for had it been otherwise an 

 explanation of it would have appeared in works of 

 chemistry, the authors of which profess ignorance of its 

 meaning. It is clear, therefore, that what is known can 

 only be gathered by inference from writers who have 

 interested themselves in the matter, and these, as far as 

 my reading has gone, have attributed to Newton the origin 

 of the scale. Mr. Cleveland Abbe gives the figures of 

 these two celebrated men as I have done, and it seems a 

 most remarkable coincidence, should they both have started 

 from the same point between freezing and boiling, and 

 adopted the same number, that they sliould have done so 

 from perfectly independent considerations. It would be 

 remarkable too, if Fahrenheit had never seen or heard of 

 Newton's instrument, invented a few years before. We 

 have only to multiply Newton's figures by eight, for the 

 reasons already given, to obtain Fahrenheit's scale. The 

 scale of the latter, as your correspondent says, in his first 

 constructed thermometer, was plus 90 and minus 90, but 

 it is not clear how this could have assisted in the framing 

 of his subsequent scale, as the numbers did not refer to 

 freezing or boiling points. That thermometer, as is known , 

 he discarded, and then made another, taking for his fixed 

 point the temperature of the human body. This was 

 Newton's method, by which he had worked. It seems to 

 be correctly said that Newton was in doubt as to a fixed 

 point for boiling, for he speaks of boiling and violent 

 boiling, as if they implied different degrees of heat. 



Newton's paper appeared in the PhUoguphical TrauMtc- 

 tions for 1701, and although anonymous has always been 

 attributed to our great philosopher. It is so said hy 

 Brewster, by Kelvin, by Renou and others. Newton 

 begins by saying that he used linseed oil in a glass tube^ 



