

COLLIMATOK. 



v| of the instrument, similar to that at the masons Uv*I ; 3. For 

 MH-iV instnuwoU, a power u( determining the relation of Uw direc- 

 tion of that object to vertical UM. Now new object oannot bo 

 seen uo UM wire* of telescope when they are in Ui foous of the 

 object glass, tod a dutuil object u very seldom sufficiently steady or 

 sharply defined. Thu want may be supplied by a second telescope, 

 having iu axis parallel to the axia of the Uleeoope under examination 

 and nearly in UM auae right line, which hat cross wires in iu focus ; 



and nearly in the aune right line, 



UM object gUewe being toward* each other. As parallel rays falling 

 on an objt gla converge to the foetu, so ray* diverging from the 

 focus become parallel after refraction at the object gUuw, and emerge a* 

 if they came from a real object at an infinite distance ; hence the cross- 

 wire* in the euppwmeoUry or collimating teleeoope will be eeen dis- 

 UncUy in the dineHo* of the line joining the oroe and the centre of 

 UM ubject-glaa.. in whatever part of the cylinder of iatuing rays the 

 eye may be placed. Great cere U requiiite in adjusting the wire* of 

 the ~i"ti g Ulesoope exactly to foous, especially if a abort teleeoope 

 be need ; but the axe* of the two telescopes need only be approxi- 



maUJv in a right line. 



In many of the private observatories in England, a metal plate with 

 aharp line* or doU engraved upon it, U firmly secured to an outside 

 stone and viewed through a lens fixed in the wall of the observatory, 

 UM disUnoe between the lens and the mark being equal to the focal 

 tirttin-* of the lens. It is evident that such a mark may be used for 

 determining the error of collimation in altitude of a reversible circle, 

 and in all oases where merely a distinct and distant object is required. 

 If the position of the mark be permanent, and the focal length of the 

 lens be considerable, this may be advantageously used as a meridian 

 atari- [TRANSIT] ; but then the lens should have a separate support 

 within the observatory, and the position of the mark should be 

 jealously watched and verified. Dr. Rittenhouse first made, use of 

 this substitute for a distant mark (' American Philosophical Trans- 

 actions,' vol. ii. p. 181); and we believe Dr. Maakelyne at one time 

 used an adaptation of the same principle, namely, a cap with a lens of 

 long focus, slipping over the object end of his transit telescope, to 

 view the south meridian mark at Greenwich, which was too near the 

 obseMiluit to be seen distinctly. 



The coUimating telescope and its cross-wires are thus made to 

 supply the want of a distinct, distant, and immovable object. In the 

 ' Astronomische Nachrichten,' No. 43, Professor Gauss, after enuncia- 

 ting the optical property above mentioned, used it for measuring the 

 intervals between the wires of a transit telescope by a theodolet, which 

 viewed them through the object-glass of the transit. In No. 61 of the 

 same work, Professor Bessel applied the same principle to a still more 

 important purpose, that of determining the horizontal flexure of the 

 telescope of his meridian circle. After taking out the object-glass and 

 eye-piece of this instrument (or the instrument might have been 

 raised out of the way), he placed two collimating telescopes, one to the 

 north and the other to the south of the circle, looking into each nth. -i . 

 and nearly in the horizontal line which passed through the centre of 

 his instrument. These he adjusted to have their cross- wires appa- 

 rently upon each other, when the two object-glasses and the two 

 crosses are evidently all in .the same right line. The object-glass and 

 eye-piece were then replaced in the circle telescope, and the angle 

 between the two rspnsm of the collimator measured, which would have 

 been exactly 180, without flexure; hence the difference from 180 

 was the double horizontal flexure of the circle telescope. Bessel 

 further remarks, that a vertical telescope turning freely round in its 

 collars, and having a cross level attached, might be used for deter- 

 mining the true zenith point of any instrument, without reversing the 

 Utter. The date of this publication is July, 1824. 



Captain Kater, who had not heard of either of them memoirs, gave, 

 in the ' Philosophical Transactions,' 1825, p. 147, a description and 

 figure of a koritontal Jioatitty collimator. This is a telescope laid 

 horizontally upon a block of cast iron, which floats in a vessel filled 

 with mercury. This collimator was designed for determining the 

 anil* point of mural and other irreversible circles. The cross of the 

 collimating telescope is observed by the circle telescope in one direc- 

 tion, suppose to the north, and the divisions read off. The trough of 

 mercury with the collimator floating in it, is then transported to the 

 south of the circle, the cross again bisected, and the divisions read off 

 as before. If the angle which the line of sight of the collimating 

 telescope makes with the horizon be supposed to be unchanged by this 

 change of place, it is clear that half way between the means of the two 

 set* of readings is the reading corresponding to the vertical position of 

 the circle telescope. Again, as the difference of the north and south 

 mean readings would equal 180, if the coUimating telescope were truly 

 horizontal, half the excess of this difference above 180, or half the 

 defect from 180', will bo the angle which the collimating telescope 

 makes with the horizon. We believe however that, in addition to the 

 trouble of moving such an apparatus, the permanence of the position of 

 UM oolUmating telescope could not be relied upon if at all disturbed. 



In the ' Philosophical Transactions,' 1828, p. 257, Captain Kater 

 proposed a very much-improved form of this instrument, whi.h h.- 

 called the rrrticul floating collimator. The iron float is here a ring 

 Hwimming in an annular trough, and the telescope, which is placed 

 vertically, has a clear view through the centre of the float and trough. 

 This collimator may be placed below the instrument to be examined, 



when UM i^Jljtn.tffig telescope will have its object-glass uppermost, or, 

 as is most usual, above the instrument, when the eolumating tele- 

 scope looks downwards. A smooth rotatory motion n|...n rollers can 

 be given to the annular trough, when it U evident the line of sight of 

 the coUimating telescope will either be and continue to be vertical 

 (supposing the position of the float to be permanent), or will describe a 

 conical surface of which the axis is vertical. Hence, if the cross be 

 bisected in two opposite positions of the collimator by the telescope of 

 a circular instrument, the mean of the two readings will be the reading 

 of the zenith of the instrument. 



It will, generally speaking, be convenient to adjust the axis of the 

 i-ollimating telescope truly vertical. To do this first observe tin- 

 position of the cross by a circle or transit telescope, turn the eolli- 

 motor half round, and note the position again ; then, by placing a 

 small weight upon the float, bring the cross half way between the 

 two observed positions. Turn the collimator a quarter round and 

 perform the same adjustment for this and its reversed situation. The 

 axis of the collimating teleeoope is now truly vertical From some 

 trials, which however we must admit were not made under favourable 

 circumstances, we do not think the vertical floating collimator capable 

 of giving results as accurate as may be obtained by other means ; but 

 it ought also to be stated, that there is a good deal of difference of 

 opinion among practical astronomers upon this point. 



In the accompanying figure we have represented a more portable, 

 and perhaps a more accurate instrument for determining the error of 

 collimation, and also the position of the horizon, than either of the 

 floating collimators. 



The three parts of which this collimator consists have been sepa- 

 rated from each other for easier comprehension. The telescope o 

 rests with its ground cylindrical collars, a a, 6 b, in the rectangular v's, 

 A, B of the stand. These collars should be truly cylindrical, and, if 

 possible, exactly equal. There are cross-wires which must first of all 

 be placed correctly in the focus of the object-glass, when the screw e is 

 to be tightened. To adjust the cross-wires bring the intersection of 

 the cross to bisect any distinct and immovable object (the wires of 

 another telescope, for instance), turn the telescope half round in its 



Ltvel Collimator, Troughton and Simms. 



v's, and then, by releasing one of the four adjusting screws (the heads 

 of which are seen near 66), and screwing up iu antagonist, bring the 

 cross half mty back to coincidence, and complete the coincidence by 

 screwing 8. When this has been done satisfactorily, adjust the cross 

 in the transverse direction by the other two screws, and it will then be 

 found that the telescope can be turned round, without any apparent 

 change of place in the cross-wires, that is, the line of sight is in the 

 axia of the collars or parallel to the axis. The reflector B, which is 

 merely to throw the light of the sky or a lamp upon the wires, may now 

 be put on. 



The collimator being thus adjusted, is to be set to the north or south 

 of the circle under examination, and at the same height as the 

 of the telescope, when the axis of the collars is to be made horizontal 

 by the reversible level LI, and the foot screw s. When the cross of 

 the collimator is bisected by the wires of the circle telescojm, tin- 

 telescope is horizontal, and the mean reading of the circle microscope is 

 the reading of the horizontal point, which, if the circle reads altitudes, 

 should be 0, and if zenith distances, should be 90. The ditl> 

 from these values ia the error of collimation. By setting the collinm- 

 tor to the other side of the instrument any error of flexure may be 

 detected. 



We have said that the evliiidru':il collars should ! perfectly equal, 

 Imt it is not easy to make them so. The difference is easily ascer- 

 tained by reversing the telescope in its r's, end for end, and again 

 ,i|H'K ing the level. Suppose the level to have shown perfect h< 

 tality before reversing, and that afterwards the reading towards o 



exceeds that towards s, by m*. It will easily be seen that must 

 always be lubtractcd from the indications of the level towards o. It is 



