260 Collimation Error of Astronomical Instruments. [Mat, 



The above readings of the reflecting collimator are the result of three 

 measures occupying at most about as many minutes to make ; and the 

 collimation error by inversion is from one inversion only. As regards 

 the wants of the amateur astronomer in India, the reflecting collimator 

 will I apprehend be eminently serviceable, if (as is very often the case) 

 the level attached for levelling the axis is dull in its movements, or 

 should it unfortunately be broken ; and should moreover the observer's 

 situation preclude the erection of a mark to examine the collimation 

 error — nothing more is necessary than a basin of quicksilver and au 

 eye-piece fitted up as above. 



We will suppose that on looking into the eye-piece the centre 

 wire and its image are both seen, and that the reflected image 

 appears 10 diameters of the wire by estimation to the east of the 

 direct image ; this may arise from error of level or error of collimation, 

 or from both ; to decide this question, we must invert the axis and 

 again estimate the distance between the direct and reflected images 

 of the centre wire — suppose the reflected image to be now situated 

 6 diameters of the wire to the west of the direct image : we have, 



Si p\ 3f_ " c reckoning .j. for eastern and — for western devi- 

 ation : from the sum we find L = -j- 1 . 

 difference, C — 4. 4. 



Shewing that the east end of the axis is too high by a space corre- 

 sponding to the thickness of the wire, and that the centre wire must 

 be moved towards the east four times its thickness. Other instances 

 might be adduced of the efficiency of the reflecting collimator, but the 

 above will I apprehend be considered sufficient. 



A mere glance at the accompanying figure will explain all that is 

 necessary to the construction, which I need hardly remark can be per- 

 formed by any common workman. 



Reflector, full size. 



Madras Observatory, ~| 

 5th April, 1835. J 



[The elegance, the simplicity, and the great practical accuracy of the method 

 described above by the Madras astronomer, will we have no doubt recommend it 

 to very general adoption. — Ed.] 



