442 



NA TURE 



\_Sept. 5, 1889 



But 'it fnay be asked where is the great difference 



^between this system and the old, as the positions of the 



star images on these photographs have to be measured 



seriatim, just as the stars themselves have to be measured 



-serimim in the old system. This is no doubt true, but 



^iMi'^'er what different conditions are the measurements 



made. 



None but those who have worked in this field know 

 the labour represented by a volume of 5000 star places 

 under the old system. 



How many times does that wretched bit of cloud come 

 across the field just as the star reaches the centre wire of 

 the micrometer, and how many nights, beautiful and clear 

 as they may look to ordinary individuals, prove utterly 

 worthless for observing purposes ; but under the new 

 system, once a single good plate has been obtained, there 

 is a permanent record of some hundreds, or in some cases 

 thousands, of stars, which can be measured at leisure by 

 day or by night, in good weather or bad weather, and in 

 comfort in your office or study, and there also is that 

 photograph as a permanent record which can be referred 

 to at any time as a check on errors which might possibly 

 creep in to some of the final reductions. 



In fact, this new system gives us the means of taking 

 advantage of the very i^^ really favourable opportunities 

 of observing, and of producing, during those favourable 

 moments, ?i facsimile, so to speak, of any portion of the 

 heavens which we can examine and survey at our leisure 

 without any of the difficulties or discomforts attendant 

 upon direct astronomical observations, and under con- 

 ditions far more favourable to the obtaining of accuracy 

 in the results. 



Let us now consider what are the chief points to be 

 attended to in the construction of the instrument. It 

 is evident that what is required is an instrument which — 



(i) Can be accurately pointed at any given object. 



(2) That when pointed in the desired direction the 

 clockwork shall cause it to follow that object as steadily 

 as possible. 



(3) That as the meridian is the best position for observa- 

 tion, the instrument should be capable of working for 

 some distance on each side of the meridian without 

 reversing. 



(4) That means should be supplied by which the observer 

 can watch and verify the accuracy of the clock-driving, 

 or making any change in position rendered necessary by 

 redaction. 



The first point involves delicate and accurate slow 

 motions in right ascer(sion and declination. 



The second involves great steadiness and rigidity of 

 the mounting, great smoothness of motion of the polar 

 axis in its bearings, and above all most accurate clock- 

 driving motion. 



The third point involves either the adoption of the old 

 English form of mounting, or a modification of the 

 German form, as the latter does not generally allow of 

 motion for any considerable extent beyond the meridian 

 without reversal. 



The fourth merely involves a very powerful finder— 

 or, rather, guiding— telescope with suitable micrometric 

 eye-piece arrangements. 



Respecting the relative merits of refractors and reflectors 

 for this purpose, I shall speak just now ; for the present 

 I wish to direct your attention to the instrumental part 

 only, and, for the better understanding the peculiarities 

 of the various mountings, I will now throw photographs 

 of some of them on the screen. 



The first illustration is that of the Paris equatorial, 

 with which the well-known and deservedly praised 

 star pictures of the Messrs. Henry were produced. You 

 see it is of the construction generally known as the 

 " English " equatorial,— a long split polar axis, with 

 bearings for carrying the telescope at the centre (its 

 weakest point). This form of equatorial has many 



advantages, which at first seem to render it peculiarly fit 

 for this special work. With good slow motion, and a 

 large finder telescope, it admirably fulfils the first, third, 

 and fourth of the above conditions ; but, owing to its 

 peculiar form, and the difficulty of using a driving sector 

 of long radius, it is not well calculated to fulfil the second 

 and most important of all conditions, viz. the accurate 

 following. No doubt excellent work has been done with 

 it by what is called the " eye and hand " guiding. That 

 is, the star is watched in a powerful finder, and when it 

 is seen to err sensibly in position, it is brought right again 

 by the slow motion handle. I think it is generally allowed 

 that the undoubted excellence of the work done by the 

 Messrs. Henry is due more to extraordinary patience and 

 skill in the " eye and hand" guiding than to any unusual 

 perfection of the clock-driving. This form of instrument 

 also possesses a disadvantage in being very difficult to 

 arrange for work near the Pole. 



The second illustration is of Dr. Gill's 9-inch photo 

 telescope, with which he took his star pictures. Tliere 

 is nothing peculiar about the mounting of this, being in 

 fact an old equatorial which I sent him some years since. 

 In this case also the excellence of his results is to be 

 attributed to the skill of the observer, and not to any 

 inherent excellence of the clockwork ; but that excellent 

 work has been done with it is apparent from the perfection 

 of some of his photographs. 



The next illustration is that of the telescope of Mr. 

 Isaac Roberts, of Liverpool, for whom I mounted a 20-inch 

 reflector to experiment with. As you see, it is mounted 

 on what I call the twin form, as I mounted Dr. Huggins's 

 instrument, by the adoption of which the observer always 

 has a second telescope available for visual work, if any- 

 thing interesting should appear in the heavens. With 

 this instrument Mr. Roberts, working in the exceptionally 

 wretched atmosphere of Liverpool, has secured some 

 most admirable work, a few specimens of which I now 

 throw on the screen. 



This is the first instrument, as far as I know, in which 

 a successful attempt has been made to drive, for any 

 considerable time, without " eye and hand guiding." The 

 special system of clockwork used I will describe further 

 on. So far as the general form of the instrument is con- 

 cerned, it would appear that the balance of advantage 

 lies with the German form (so called) — that usually 

 adopted in this country. It is capable of being made of 

 great stability, and it admirably fulfils all the conditions 

 except the third, but that also, by a little modification, 

 can be managed. 



The next illustration shows the general form which 

 I have adopted, the principal feature of which is great 

 stability. The stand being cast all in one piece con- 

 tributes to this, but the peculiarity of the system of 

 equipoise probably has more to do with it. In de- 

 signing these instruments, I projiosed that the fulcrum 

 of the levers which support the greater part of the 

 weight of the polar axis, should be attached, not to the 

 frame of the instrument, but to an independent pillar, 

 so that only a very small portion of the weight of the 

 moving parts should be carried by the main framing. 

 Dr. Gill then proposed that I should also allow the levers 

 to act in a purely vertical direction, instead of, as usual, 

 in a direction at right angles to the polar axis, and to let 

 the point of support be vertically under the centre of 

 gravity of the whole moving part. This I have carried 

 out, and the result is that only as much of the weight of 

 the whole instrument as is necessary to insure steadiness 

 will rest on the bearings (lateral or end bearings) of the 

 polar axis ; all the rest is transferred to the base of the 

 stand. 



We now come to consider the all-important part of the 

 photographic equatorial — that is, the driving clock. 



All clocks used for driving equatorials (which must of 

 course move uniformly, and not step by step as pendulum 



