376 



♦ KNOWLEDGE ♦ 



[JuKB 22, 1883. 



LIMITS OF TELESCOPES. 



IT startles most people to be told they never see the 

 stars, and yet in an important sense this is quite true. 

 Tlie human eye, or a telescope, forms an optical image 

 with the light rays rccei\('d from the stars, but not a 

 picture. The stars are too far ofl" for their light to bring 

 to us the smallest visiljle representation of their surface, 

 and the discs which a well-corrected telescope shows are 

 only optical images. If, on a clear, calm night, when the 

 air is steady, a telescope is pointed at a planet, it forms a 

 real picture, though the distance of the olyect prevents 

 details, even of considerable size, from being seen. If, 

 after using an eye-piece that magnifies fifty times linear, 

 another of double that power is employed, the planet's disc 

 will look twice as broad. Try the same experiments 

 with a bright star, and if the air is clear, and quiet 

 enough to produce no confusion, the star disc will not 

 be made bigger by the higher power. Larger telescopes, 

 if good, show stars with lesser discs than smaller instru- 

 ments, and hence a 6-in. telescope will plainly separate 

 double stars so close that a 3-in. cannot divide them at 

 all. Looking at the moon with an ISin. telescope and 

 a power of 200, shows more details than the same magni- 

 fication can display with a 6-in. or 8-in. It is only on 

 rare occasions, and in very favourable situations, that 

 large telescopes can be used effectively with magnifying 

 powers proportioned to their size. The larger they are, 

 the more disturbed air they look through ; and it often 

 happens that, instead of showing large stars with neatly- 

 rounded discs, they exhibit them like luminous flags 

 fluttering in a strong wind. Mr. C. Wolff, writing in Ciel 

 nl 'furre, says that Signor Schiaparelli, of the Milan Observa- 

 tory, can, with a telescope by Merz, with an aperture 

 (diameter of object-glass) of 0'" -218, or about 8i inches, 

 see an object on Slars like Jutland, Cuba, or the Isthmus 

 of Panama. The Washington telescope, of 2.5 J inches 

 aperture, he states, would distinguish a round object on 

 the moon of about 344 yards in diameter, or an elongated 

 one of half that breadth. He observes, " We are far from 

 the promises made by Arago, when he asked the Chamber 

 of Deputies for a credit for a telescope of 0'" -38 (about 

 1 o inches) aperture. He thought, by forcing the magnifi- 

 cation to 6,000, a round object on the moon about 65 J feet 

 square, or a long object about 6i feet wide, might be seen. 

 He considered only the difliculty of obtaining images bright 

 enough for such magnification ; but Foucault showed that 

 great apertures were necessary for such purposes, and that 

 to reach the limits mentioned by Arago, a telescope must 

 have an object-glass or mirror nearly 10 mfetres (about 

 33 feet) in diameter. " If," says Mr. Wolff, " we could 

 make such instruments, they would be nearly useless." 

 They would be hindered from working properly by the 

 disturbances of the air. 



It is probable that large telescopes will be set up in the 

 best climates that can be discovered for affording clear, 

 steady vision ; but if a locality could be found in which the 

 moon could be continuously observed with a magnification 

 of 2,000, that would only be like bringing tlif eye of the 

 observer within about 170 miles of our satellite, and the 

 planets, being so much further ofl" could only exhibit objects 

 of very considerable dimensions. 



We give the above statements as made by Mr. Wolff, 

 but precise explanations and observations are needed to 

 settle such questions. For example, what is to be called 

 " seeing a lunar object " 1 One telescope shows a small spot, 

 without details ; a larger one exhibits in that spot a little 

 hollow ; a still larger some roughnes.sos in the hollow. Would 

 Mr. Wolff consider the object seen by the first instrument 1 



(^tutorial (gossip. 



In the Pavilion at Brighton the use of the oxy-hydrogen 

 lantern has, of late, been forbidden — a restriction rather 

 inconvenient for lecturers. It would, probably, pay well 

 to build a hall in Brighton capahile of holding as many a& 

 the Dome, with good acoustic properties (those of the 

 Dome are singularly bad), and with sensible arrangements 

 in lieu of the unwise ones in vogue at the Pavilion. The 

 charge for the Dome is higher than would Vje just for as 

 large a hall of first-rate proportions, and with the best 

 modern improvements. 



Looking over some of the later editions of Dickens's 

 works, I have been struck by the way in which some of 

 the rules apparently in vogue among compositors have 

 made nonsense of his words. Thus, it appears to be a 

 rule that the word " however " shall always be closely 

 guarded by commas. In the " Charles Dickens' Edition " 

 of " Great Expectations " this rule comes in effectively 

 to improve the conversation between Pip and Mag- 

 witch. Dickens made Pip say, " Still, however you 

 have found me out, you have found me out," <fcc., 

 and a little later Magwitch says, " However you have 

 found me out, you seys jest now. Well ! however 

 did I find you outi" Of these three " howevers," the 

 two first are, of course, provided with a sequent comma, 

 making both sentences absurd. The third escapes ; 

 wonderful to relate. True, the absurdity would have been 

 even wilder if that third " however " had had its comma. 

 But, " says the ghost, says he, vy that's no rule," on the 

 contrary, that was a raison de j}lus for adding the pre- 

 posterous comma. In more editions than one, by the way, 

 of the same story we find the absurdity, " ' His wish were ' 



said Joe as I would, and you that,' " — for " His wish 



were as I would ' and you that." 



One of my own favourite hetes noirs in this line is the con- 

 stant custom of adding a comma after " and " in sentences 

 beginning " And therefore," or the like. Imagine reading 

 or speaking with a pause after the " and " in such cases ! 

 Of course the sense is impaired as well as the sound ; but 

 the sound should be a sufficient guide. 



Throwing in a comma before " which " often produces 

 a most absurd change in the meaning of a sentence, while 

 the sound in reading is not impaired, so that in correcting 

 proofs one is apt to overlook the change. For instance, 

 speaking of the crescent phase, I wrote, " We recognise 

 this peculiarity in the planets which travel nearer to the 

 sun than our earth does." This does not read better (or 

 worse) than " We recognise this peculiarity in the planets, 

 which travel nearer to the sun than our earth does ; " but 

 it happens to be a trifle nearer the truth. 



The valuable series of papers on the " Moon," by our 

 esteemed contributor, " F.R.A.S.," must for a time, which 

 we trust may be short, be suspended. 



Referring apparently to eclipse expeditions and the 

 theoretical views which they either have or have not 

 confirmed, the Glasgow Herald (in a review of my " Light 

 Science for Leisure Hours ") has the following impressive 

 passage, 



(" Which is pretty, but I don't know what it means.") 

 " There was bound to be a little too much shouting over 

 realisations out of those unknown regions, and there will 



