328 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



LOclOBEB, 



at riffht angles to the vertical axis on which the telescope turns, it 

 is of little consequence whether or not the line of si;jht he pre- 

 cisely '" directum with the centre line of the lenses. The amount 

 of distinctness occasionally lost hy the want of this coincidence 

 is altogether inappreciable. 



If this then were the only difficulty attending the adjustment of 

 tlie diaphragm, we could not do better than, pointing the instrument 

 at a placard lettered with various type at some distance, move the 

 diaphragm up or down, until the horizontal wire appearing in 

 centre of the field might be seen to intersect those letters most 

 distinctly defined. The adjustment required is, however, of a 

 different nature. Mechanical error would almost unavoidably, 

 during focal adjustment, cause the focu.s of the eye-piece to deviate 

 from the path of a line of sight so determined upon, sihce it is 

 very doubtful whether it would be possible to construct tubes to 

 slide one within the other with the nicety which this would require. 

 This source of error, with a remedy, was first pointed out by 

 Mr. Gravatt ; hut his method seems (as I shall show) to be at 

 least liable to misinterpretation, if not capable of improvement, 

 and perhaps correction. The ol)ject of his adjustment he defines 

 to he, "to examine and correct the line of collimation." Had he, 

 instead of this, described it as a process to correct error arising 

 from focal adjustment, mistake on this head would not have been 

 so likely to occur. Having described the process, to which I shall 

 have occasion again to reveit, he adds: "The instrument will now 

 be in complete practical adjustment for level, curvature, and hori- 

 zontal refraction, for any distance not exceeding 10 chains, the 

 maximum error not being more than , ^'^ ,^ th part of a foot." This 

 might perhaps have been with advantage omitted, as the three 

 sources of error alluded to in this paragraph, remain unaltered by 

 the adjustment described — the same exactly whether the adjust- 

 ment had taken place or not. 



1°. The steps of this adjustment involve the following principle: 

 tliree stakes, A B C, are driven in to the ground, equidistant, and 

 tops in a curve of true level. Set the instrument up at A, using 

 the bubble merely to see that you do not disturb the instrument; 

 take the readings of the staff held on each stake consecutively, 

 and if the difference of the readings at A and C be four times the 

 difference of the readings at A and B, the line of sight is unal- 

 tered by focal adjustment, on the principle that the radiating 

 differences between true and apparent level vary as the squares of 

 tlieir respective distances from the point of contact of the tan- 

 gent and curve; if not, alter the diaphragm till this proportion 

 take place. 



2^. Next, lower or elevate the telescope by means of the parallel 

 plate-screws until the line of sight so adjusted and apparent level 

 coincide, and set the bubble parallel to it. 



3'. Get the line of sight and bubble at right angles to the ver- 

 tical axis of the telescope in the usual manner. 



With regard to No. 1, which from its importance deserves 

 most consideration, since the bubble is used merely to see that 

 you do not disturb the instrument, the line of sight may be a 

 secant to the curve marked on the ground, and not a tangent, 

 which circumstance might negative this adjustment without due 

 precaution; but if the bubble and optical axis are nearly parallel 

 when tlie instrument is obtained from the optician, and the bubble 

 be brought to the centre of its run previous to taking the readings, 

 I do not believe sensible error likely to accrue from this source. 



The manner in which focal error is got rid of may be ex- 

 plained as follows: Suppose the instrument placed, the set in 

 apparent level, and a staff held vertically at a considerable dis- 

 tance, ai)pearing in the centre of the field of the telescope. Sup- 

 pose, further, this staff to advance or recede, without altering its 

 relative position to the optical axis; a single point of the image of 

 the staff will travel backwards or forwards horizontally within 

 the tube; the ])aths of the remaining points will all be less or more 

 inclined to the optical axis, forming every variety of angle with it, 

 according as they are nearer to or further from this normal point. 

 Suppose, further (whicli is more than probable), the tube carrying 

 the eye-piece not to slide in directum with the optical axis, but in 

 another line, we must then raise or depress the diaphragm out of 

 the optical axis, till the cross hair meet a ray from the staff whose 

 ath shall coincide with that of the focus of the eye-piece during 

 ocal adjustment. This is evidently possih/e, and no method better 

 to effect this than Mr. Gravatt's, with the precaution already 

 mentioned. 



This adjustment, once made, need never be repeated. The 

 remaining two adjustments may tlien be performed in the usual 

 easy manner. 



[: 



There is, however, considerable grounds for supposing misap- 

 prehension to exist on this head among many who practise and 

 some who write. I shall conclude these remarks with a specimen 

 of the latter kind, taken at random from a text-book lately pub- 

 lished, where, speaking of Mr. Gravatt's method, the writer says: 

 "We are indebted to Mr. Gravatt, of whose level we shall here- 

 after speak, for a method of collimating which satisfies the above 

 requirements, and removes any error arising from imperfection ia 

 the slide of the telescope, while at the same the line of collima- 

 tion is set with the end at the object glass slightly depressed, 

 instead of exactly horizontal, so as to remove, or nearly so, the 

 errors arising from the curvature of the earth, and the horizontal 

 refraction." 



Cirencester, 



J. D. Pembebton. 



BRITISH ASSOCIATION. 



Selections from Papers read at the Meeting held at Edinburgh, 



August, 1850. 



CContinued from page 304.) 



Description of a New Arrangement of Refecting Telescope, by which 

 much comfort and convenience is secured to the Observer. By 

 James Nasmyth. 



In introducing this subject to the attention of the members of 

 the Mechanical Section, Mr. Nasmyth, with a view to render the 

 description of his improved arrangement of telescope more clear 

 to such members as might not be practically conversant with the 

 subject in question, premised his description by a sketch of the 

 various forms of reflecting telescope which had hitherto most 

 generally been in use. These are seen in fig. I, 2, 3; fig. 1 being 



c<jr 



Fig. 1.— Nbwtokiak. 



m 



Vig. 2. — OBBflORIAK. 



Fig. 3.— Cabsegrain. 



r 



_m_ 





Fig. 4. — Nasmyth. 



the Newtonian, which is the arrangement most generally in use for 

 the larger and most powerful instruments. It will be seen, that in 

 the case of fig. 1, the object is viewed by the observer placing his eye 

 at the side of the tube, and at the end most distant from the spe- 

 culum S,the image of the object being seen in that direction by means 

 of the employment of the small diagonal plane mirror at B. In 

 telescopes of this construction, the eye of the observer is placed 

 near the upper end of the telescope; thus, fig. 6. It is therefore 

 requisite that he must change his situation almost constantly, so 

 as to follow with the telescope the movement of the star, or 

 other astronomical object he is desirous to look at. In telescopes 

 of a moderate size, this may not be found to be a very serious 

 source of inconvenience; but when we come to employ instruments 

 of this class, of a larger and more powerful description, the diffi- 

 culty of following the eye-piece of the instrument, when its posi- 



