Sig. Santini on Achromatic Telescopes. 113 



second powers of the corrections found, would have been eva- 

 nescent; the last, that is the spherical aberration in the red rays, 

 is about the same order as that which remains in the double 

 object glasses, constructed according to the theories of Her- 

 schel and Frauenhofer ; and therefore less than what remains 

 in the triple object-glasses. It cannot be taken away altoge- 

 ther, but may still be diminished, if it would be worth while to 

 give up the simplicity of construction which lenses of equal 

 curvature present, such as we have supposed the first and the 

 second to be. But it is evident, at the same time, that the aber- 

 rations vary considerably with the rays of the last lens ; and 

 hence the construction of a correcting lens of this sort requires 

 great care, and should not be recommended, except for the 

 greater telescopes designed for astronomical observations, for 

 which they may be employed with great advantage and saving 

 of expense. Another great advantage from the construction 

 of Mr. Rogers is, the field being free from a coloured edge. 

 In fact, the condition that should have place in a system of 

 three lenses separated by the distances d, d' } and disposed 

 round the same axis, that the coloured margin should be 

 removed, is represented by the equation (Vol. ii. No. 246) 



o £, which in this case is identical, since d' = 0; q+r=0. 



Such a combination, moreover, deserves all the attention of 

 practical opticians. 



Giovanni Santini. 



Travels in Turkey, Egypt, Nubia, and Palestine, fyc, in 

 1824, 5, 6, and 7. Byll. R. Madden, Esq., M.R.C.S. 2 vols. 

 London, 1829. 



At a moment when political events, of a magnitude scarcely to 

 be estimated, are rivetting the attention of the world upon 

 Turkey and Turkish interests, comes forward this, among 

 other works, calculated to satisfy the literary craving which 

 such a state of things is sure to engender. But, independent 

 of the interest which Mr. Madden's work possesses from the 

 peculiar circumstances of the times, it is well worthy of atten- 

 tion from the curious and obviously faithful picture which it 

 JUNE — sept., 1829. I 



