4 



Dr. B. Stewart on an Apparatus [June 20, 



the telescope. This adjustment is one which, from the form of the two 

 collimators (a straight line and two cross lines), can be made with great 

 exactness, and when accomplished, it follows that the two collimators, 

 F and Gr, are to be optically regarded as two infinitely distant bodies, 

 both being in the same direction. 



Each of the two collimators has a moveable cover, so that, if desirable, 

 the one can be viewed without the other. 



The collimator lines are illuminated in the following manner: — These 

 lines are formed of fine glass threads, and the light of candles symme- 

 trically disposed is allowed to fall upon these threads. By this arrange- 

 ment the threads are rendered luminous on both their sides at the same 

 time ; there is therefore no perceptible parallax, such as would follow 

 from the one side of the thread being lit up at one time, and the other at 

 another time. 



It is now necessary to describe the method of fixing the collimators. 



A brick erection was made in the basement hall of the Observatory, 

 having the shape of a circular arc. To the flat top of this erection three 

 pieces of slate, all in one horizontal plane, and having their upper sur- 

 faces curved, were attached by cement ; finally, a slate slab, E (shown 

 in the figure), was laid so as to rest simultaneously on these three curved 

 surfaces. The collimators, being intended to rest on this slate slab, had 

 their lower surfaces made quite flat, and were firmly bolted by means of 

 screws to the slab. 



The angular distances between the collimators are (roundly speaking) 

 as follows : — 



Erom 1 to 2 30° 



„ 1 „ 3 60° 



„ 1 „ 4 105° 



„ 1.5 120° 



A horizontal table, B, capable of motion, either vertically or in 

 azimuth, and also capable of being rigidly fixed in any required posi- 

 tion, is placed in the centre of the circle of which the boundary line of 

 the slate slab constitutes the circumference. 



In order to determine accurately the angular distance between the 

 various collimators, a theodolite is placed on the table B, so that when 

 levelled its telescope, as it sweeps round in azimuth, may be able to bring 

 into the middle of its field of view the various collimators. 



No care need be taken that the centre of the theodolite is precisely in 

 the centre of the circle, because the collimators being virtually at an 

 infinite distance, it follows that the angular distance between any two 

 of them does not depend on the exact centering of the theodolite. 



Now, if any theodolite bq taken, and if a number of sets of observa- 

 tions of the angular distances between the collimators be made, each 

 set starting from a fresh point in the azimuth circle of the theodolite, it 



