6o4 



NATURE 



[October 22, 1891 



would find out, if he came next morning, that the work was by 

 no means over. By far the largest part has yet to follow. An 

 observation that requires only two or three minutes to make at 

 night, requires at least half an hour for its reduction by day. 

 Each observation is affected by a number of errors, and these 

 have to be determined and allowed for. Although solidly 

 founded on massive piers resting on the solid rock, the con- 

 stancy of the instrument's position cannot be relied upon. It 

 goes through small periodic changes in level, in coUimation, and 

 in azimuth, which have to be determined by proper means, and 

 the corresponding corrections have to be computed and applied ; 

 and, also, there are other corrections for refraction, &c., which 

 involve computation and have to be applied. But these matters 

 would fall more properly under the head of a special lecture 

 upon the transit instrument. I mention them now, merely to 

 explain why so great a part of an astronomer's work comes in 

 the daytime, and to dispel the notion that his work belongs only 

 to the night. 



One might very well occupy a special lecture in an account of 

 the peculiarities of what is called personal equation — that is to 

 say, the different time which elapses for different observers 

 between the time when the observer believes the star to be 

 upon the wire, and the time when the finger responds to the 

 message which the eye has conveyed to the brain. Some ob- 

 servers always press the key too soon ; some always too late. 

 Some years ago I discovered, from observations to which I will 

 subsequently refer, that all observers press the chronograph key 

 either too soon for bright stars or too late for faint ones. 



Other errors may, and I am sure do, arise both at Greenwich 

 and the Cape, from the impossibility of securing uniformity of 

 outside and inside temperature in a building of strong masonry. 

 The ideal observatory should be solid as possible as to its 

 foundations, but light as possible as to its roof and walls — say, 

 a light framework of iron covered with canvas. But it would 

 be undesirable to cover a valuable and permanent instrument in 

 this way. 



But here is a form of observatory which realizes all that is 

 required, and which is eminently suited for permanent use. 

 The walls are of sheet iron, which readily acquire the tem- 

 perature of the outer air. The iron walls are protected from 

 direct sunshine by wooden louvres, and small doors in the iron 

 walls admit a free circulation of air. The revolving roof is 

 a light framework of iron covered with well-painted papier- 

 mache. 



The photograph now on the screen shows the interior of the 

 observatory, and this brings me to the description of observa- 

 tions of an entirely different class. In this observatory the roof 

 turns round on wheels, so that any part of the sky can be 

 viewed from the telescope. This is so, because the instrument 

 in this observatory is intended for purposes which are entirely 

 different from those of a transit circle. The transit circle, as 

 we have seen, is used to determine the absolute positions of the 

 heavenly bodies ; the heliometer, to determine with greater 

 precision than is possible by the absolute method the relative 

 positions of celestial objects. 



To explain my meaning as to absolute and relative positions. 

 It would, for example, be a matter of very little importance if the 

 absolute latitude of a point on the Royal Exchange or the Bank of 

 England were one-tenth of a second of arc (or lo feet) wrong in 

 the maps of the Ordnance Survey of England — that would con- 

 stitute a small absolute error common to all the buildings on the 

 same map of a part of the city, and common to all the adjoining 

 maps also. Such an error, regarded as an absolute error, would 

 evidently be of no importance if every point on the map had the 

 same absolute error. There is no one who can say at the present 

 moment whether the absolute latitude of the Royal Exchange — 

 nay, even of the Royal Observatory, Greenwich — is known to 

 10 feet. But it would be a very serious thing indeed if the re- 

 lative positions on the same map were lo feet wrong here and 

 there. For example, if of two points marking a frontage 

 boundary on Cornhill, one were correct, the other lo feet in 

 error, what a nice fuss there would be ! what food for lawyers ! 

 what a bad time for the Ordnance Survey Office ! Well, it is 

 just the same in astronomy. 



We do not know, we probably never shall know with cer- 

 tainty, the absolute places of even the principal stars to one-tenth 

 of a second of arc. But one-tenth of a second of arc in the 

 measure of some relative position would be fatal. For example, 

 in the measurement of the sun's parallax an error of one-tenth 

 of a second of arc means an error of i,coo,ooo miles, in round 



NO. II 4 7, VOL. 44] 



numbers, in the sun's distance ; and it is only when we can be 

 quite certain of our measures of much smaller quantities than 

 one-tenth of a second of arc, that we are in a position to begin 

 seriously the determination of such a problem as that of the 

 distances of the fixed stars. For these problems we must use 

 differential measures — that is, measures of the relative positions 

 of two objects. The most perfect instrument for such purposes 

 is the heliometer. 



Lord McLaren has kindly sent from Edinburgh, for the pur- 

 poses of this lecture, the parts of his heliometer which are 

 necessary to illustrate the principles of the instrument. 



This instrument is the same which I used on Lord Crawford's 

 expedition to Mauritius in 1874. It was also kindly lent to me 

 by Lord Crawford for an expedition to the I.-land of Ascension 

 to observe the opposition of Mars in 1877. In 1879, when I 

 went to the Cape, I acquired the instrument from Lord Craw- 

 ford, and carried out certain researches with it on the distances 

 of the fixed stars. 



In 1887, when the Admiralty provided the new heliometer 

 for the Cape Observatory, this instrument again changed hands. 

 It became the property of Lord McLaren. I felt rather dis- 

 loyal in parting with so old a friend. We had spent so many 

 happy hours together, we had shared a good many anxieties 

 together, and we knew each other's weaknesses so 'well. But my 

 old friend has fallen into good hands, and has found another 

 sphere of work. 



The principle of the instrument is as follows. [The instru- 

 ment was here explained.] 



There is now on the screen a picture of the new heliometei 

 of the Cape Observatory, which was mounted in 1887, and has 

 been in constant use ever since. It is an instrument of the most 

 refined modern construction, and is probably the finest apparatus 

 for refined measurement of celestial angles in the world. 



[Here were explained the various parts of the instrument 

 in relation to the model, and the actual processes of observa- 

 tion were illustrated by the images of artificial stars projected on 

 a screen.] 



Here, again, there is little that conforms to the popular idea 

 of an astronomer's work ; there is no searching for objects, no 

 contemplative watching, nothing sensational of any kind. On 

 the contrary, every detail of his work has been previously 

 arranged and calculated beforehand, and the prospect that lies 

 before him in his night's work is simply more or less of a 

 struggle with the difficulties which are created by the agitation 

 of the star images, caused by irregularities in the atmospheric 

 refraction. It is not upon one night in a hundred that the 

 images of jtars are perfectly tranquil. You have the same effect 

 in an exaggerated way when looking across a bog on a hot day. 

 Thus, generally, as the images are approached, they appear to 

 cross and recross each other, and the observer must either seize 

 a moment of comparative tranquillity to make his definitive bi- 

 section, or he may arrive at it by gradual approximations till he 

 finds that the vibrating images of the two stars seem to pass 

 each other as often to one side as to the other. So soon as such 

 a bisection has been made, the time is recorded on the chrono- 

 graph, then the scales are pointed on and printed off, and so the 

 work goes on, varied only by reversals of the segments and of 

 the position circle. Generally, I now arrange for thirty-two 

 such bisections, and these occupy about an hour and a half. By 

 that time one has had about enough of it, the nerves are somewhat 

 tired, so are the muscles of the back of the neck ; and if the 

 observer is wise, and wishes to do his best work, he goes to 

 bed early and gets up again at two or three o'clock in the morn- 

 ing, and goes through a similar piece of work. In fact, this 

 must be his regular routine night after night, whenever the 

 weather is clear, if he is engaged, as I have been, on a large 

 programme of work on the parallaxes of the fixed stars, or on 

 observations to determine the distance of the sun by observations 

 of minor planets. 



I will not speak now of these researches, because they are 

 still in process of execution or of reduction. I would rather, 

 in the first place, endeavour to complete the picture of a night's 

 work in a modern observatory. 



We pass on to celestial photography, where astrometry and 

 astrophysics join hands. Here on the screen is the interior of 

 one of the new photographic observatories, that at Paris. [Brief 

 description.] 



Here is the exterior of our new photographic observatory at 

 the Cape. Here is the interior of it, and the instrument. [Brief 

 description.] 



