Fluid Lenses. 183 



11 feet 9 inches, and with a power of 610 would divide y 

 Coronas Borealis, 0"*6, 4 and 7 magnitude. This seems to 

 justify Yon Littrow's opinion, that it might set itself boldly in 

 competition with the productions of the best workshops in 

 Europe. It would be interesting to know what may have 

 been the subsequent fate of this noble, but, as may be appre- 

 hended, most unfortunately destined instrument. The neglect 

 with which the construction has been treated in England 

 seems matter of regret. It is true that some of our first 

 opticians look unfavourably upon it, but it has never had the 

 advantage of a trial at their hands. Many of Plossl's smaller 

 instruments bore a very high character. Prof. Schumacher 

 had one of 25^- Paris lines aperture, with powers 60 and 86, 

 which would separate e Bootis, and which was thought by 

 Struve I. and himself superior to anything they had seen of 

 that size. Sir J. Herschel's opinion, too, that it is " a very 

 artificial and beautiful invention, highly deserving further 

 trial," ought surely to lead to a more practical inquiry into 

 its capabilities; and this, our readers will be glad to find, 

 has been undertaken by Mr. H. Ingall, with much prospect 

 of success.* 



The dialyte, however, was not the earliest attempt to 

 obviate the difficulty arising from the scarcity of large discs of 

 flint glass. We have given it priority, because its construc- 

 tion with glass alone follows most easily from that of the 

 common achromatic, but it had been anticipated by an in- 

 genious contrivance of Prof. Barlow. This celebrated mathe- 

 matician had perceived that, as our readers will readily 

 understand from what has been said of the dialyte, a material 

 possessing a stronger dispersion than flint glass might be set 

 back from the object-lens by a distance corresponding with its 

 chromatic power, and such a material he found in sulphuret of 

 carbon. This extraordinary fluid was just suited to his purpose 

 from its perfect transparency, absence of colour, and high 

 dispersive quality ; and to form it into a concave lens it was 

 only necessary to enclose it between two discs of glass, each 

 wrought to the requisite curve, but with parallel faces like a 

 watch-glass, so as to have no refractive or dispersive action. 

 These were applied to the two opposite sides of a third disc, 

 which being wrought to corresponding curves exactly coincided 

 with them, but whose centre was bored out so as to convert it 

 into a broad ring. The three discs and the fluid being all gently 

 warmed to a temperature higher than any likely to occur in 

 practice, the ring was placed horizontally upon one of the discs, 



* M. Vak, the ex-director of the Observatory at Marseilles, had undertaken 

 the construction of two dialytes, with apertures of about 1\ and 15^ inches j but 

 their execution was frustrated by his retirement (an unwilling one) in 1863. 



