106 Sig. Santini on Achromatic Telescopes. 



sides of the receiver ; and there melting, it will run down in 

 drops beneath the watery solution, forming a fluid flattened 

 globule at the bottom. When the receiver has become quite 

 cold, the oxide will become solid and crystallize. One such 

 operation has yielded thirty grains of the crystallized oxide, 

 besides a strong aqueous solution of it. 



On Achromatic Telescopes. By Signor G. Santini, Director of 

 the Observatory at Padua. 



Since the publication of my theory of optical instruments, 

 Mr. Rogers has read to the Astronomical Society of London, a 

 Memoir, which appears to be of the highest interest for optical 

 science, but with which I am unacquainted, except by an 

 extract inserted in vol. v. of a periodical work published at 

 Vienna, by MM. Ettingshausen and Baumgartner, entitled 

 " Zeitschrift fur Physik und Mathematik," p. 120. Mr. 

 Rogers, considering that the principal obstacle to constructing 

 achromatic object-glasses for large refractors, is the difficulty 

 of obtaining large pieces of pure homogeneous flint-glass, free 

 from striae, and thus fit, by being combined with another lens 

 of crown glass, to produce at once an achromatic lens, has en- 

 tertained the happy idea of interposing between the lens of 

 crown glass and its focus, when the pencil of luminous rays is 

 much contracted, a correcting lens composed of two smaller 

 lenses of crown and of flint glass, brought into contact, which, 

 retarding the convergence of the red rays, and removing further 

 off that of the violet rays, produces, in the rays of mean re- 

 frangibility, the effect of a plane glass ; and it is clear that 

 correct images will be produced, if the surfaces of the correct- 

 ing lens and the distance from the greater object lens be so 

 arranged, that all the heterogeneous rays parallel to the axis 

 be united in the mean focus of this last. The eminent author 

 lays down a simple rule for determining the focal distances of 

 the correcting lenses ; and then observes, that if these be con- 

 structed, nearly according to the dimensions laid down in the 

 rule, the remaining chromatic errrors may be destroyed by 

 means of a micrometrical motion, by which the two smaller 



