328 Mr Barlow on the Cmistrnction of 



or, is there not in the beginning such an irrationality ? Se- 

 condly, May there not be a great loss of light by reflection at 

 the second lens, considering the density of the rays at the place 

 of incidence ? The best reply to these questions is a reference to 

 the two telescopes already constructed, which exhibit no remark- 

 able defect of either kind*, at least that I am aware of; on the 

 contrary, with regard to the latter, the quantity of light is rather 

 in excess than in defect, compared with the usual construction. 

 Other queries relative to the ultimate success of this proposition 

 may also reasonably be anticipated ; as, for example, can the 

 fluid be permanently secured ? and, if so, Will it preserve its 

 transparency, and other optical properties ? Will it not act upon 

 and destroy the surface of the glass, &c. ? To these and similar 

 queries I reply, that, with any particular fluid which has not 

 been submitted to these trials, experience is the only test we can 

 have. Our spirit levels, spirit thermometers, &c. show that 

 some fluids at least may be preserved for many years, without 

 experiencing any change, and without producing any in the ap- 

 pearance of the glass tubes containing them. I beg, however, 

 to add, that, should any of these happen except the last, nothing 

 can be more simple than to supply the means of replacing the 

 fluid at any time, and by any person, without disturbing the ad- 

 justment of the telescope ; and the same means may be resorted 

 to (if they should be found necessary in constructing a very large 

 instrument), to prevent the external pressure of the atmosphere 

 distorting the figure of the glasses containing the fluid. Such 

 experiments as these, however, and the construction of an achro- 

 matic fluid refracting telescope, with a proper stand, on the 

 scale which I feel every confidence in being able ultimately 

 to accomplish, viz. one of at least 12 or 14 inches aperture, 

 involve expences which can only be conveniently borne by men 

 of fortune and public institutions, I have therefore done my- 

 self the honour to submit my proposition, with the results al- 

 ready obtained, to the Board of Longitude ; and if these Jlrst 

 results, although not every thing that could be wished, should 

 still be such as to prove the practicability of the Proposition, and 



* Mr Barlow has since presented a Memoir to the Rojal Society, illus- 

 trating, on theoretical principles, the tendency of this construction to destroy 

 the secondary spectrum, an imperfection inseparable from the usual form of 

 refracting telescoped. 



