34 MR. barlow's account OF THE CONSTRUCTION OF 



motions are so smooth, and the power so arranged, that it may be managed 

 by one person with the greatest ease, the star being followed by a slight touch, 

 scarcely exceeding that required for the keys of a piano-forte*. 



In the first instance I erected this stand on a platform in my garden, but I 

 soon found that exposure to the weather very much injured its action ; more- 

 over, the difficulty of mounting and dismounting the telescope was consider- 

 able, and liable to derange its adjustments. I was therefore almost under the 

 necessity of erecting an observatory to contain it. This is an excellent light 

 piece of carpentry by Mr. Smart of Lambeth, 16 feet clear in diameter, with 

 a revolving conical roof rising 9 feet above the walls. 



The roof contains 360 superficial feet, and weighs by estimation about 

 10 cwt. It is however by a simple apparatus made to revolve and open to any 

 required azimuth, by the application of a force of about 10 or 12 pounds. The 

 whole is well fitted up, and forms a neat light building, which by permission of 

 His Lordship the Master General is erected on a piece of Ordnance ground 

 adjoining my premises, commanding an entire view of the heavens for all 

 altitudes exceeding 10°. 



Having thus stated generally the nature of my operations, I shall proceed 

 to explain them more particularly under distinct heads in the following pages. 



Preliminary Experiments. 



In my former paper (Phil. Trans. 1828: Art. VII.) I have endeavoured to 

 show the effect which opening the lenses to different distances produces on the 

 secondary spectrum ; my first object, therefore, in these experiments was to 

 ascertain by actual obsei-vation the best position of the lenses for the diminution 

 of this defect. 



In order the better to classify my experiments on this head, it will be best 

 to refer to the original formula for the destruction of colour, given in my 

 paper in the Phil. Trans. 1827: Art. XV. in which I have shown, that with open 

 lenses we have, when the dolour vanishes, ^■^~ ^^^ = I. 



* I ought to state that I am indebted for the design, arrangement, and superintendence of the con- 

 struction of this apparatus to Mr. John Kingston, acting master millwright in His Majesty's Dock 

 Yard at Woolwich : a highly ingeriious.and valuable member of that establishment. 



