EXPERIMENTS ON CUAIACUM. 



axis of the telescope, and, R S, a ray passing llirougb the centre 

 of the object-glass. This ray will fall, in an uncompounded state, 

 upon the eye-glass, D, even if the object-glass were not achro- 

 matic; for, as it passes through its centre, it undergoes two 

 equal and opposite changes, and is therefore not separated into 

 the prismatic colours. After refraction through the lens, D, the 

 rays, R S, will be decomposed, and the red portion of it will 

 meet the axis in r, and the violet ray will meet the axis in v. 

 But when the second eye-glass, F, is interposed, the red ray will 

 meet its anterior surface in wz, and the violet ra}- at a point, ti, 

 nearer the axis. Now, as the refracting angle of the lens is 

 greater at m than at n, the red ray, S m, will be more refracted 

 than the violet ray, S n; and this increase of refraction will com- 

 pensate for its inferior degree of refiangibility. From this cause 

 the refraction of the second lens, E, will render the resulting 

 rays, ni/, nv', parallel, and thus destroy the chromatic aber- 

 ration, which is always proportional to the angle formed by 

 the resulting rays, m r, n v'. In order that distinct vision may 

 be obtained with this eye-piece, the rays must fall converging 

 on the first eye-glass, D. This can be effected only by 

 placing the lens, D, between the object-glass and its principal 

 focus, and by this means the telescope is rendered shorter 

 than if only one convex lens had been employed. The equi- 

 valent lens, or the lens which would produce the same magnify- 

 ing power as this eye-piece, is equal to half the focal length of 

 the glass, D.. 



I am, Sir, 

 Your obedient humble servant, 

 Mount Annan, Juli/ 21, 1806\ DAVID BREWSTER. 



V. 



Chemical Experiments on Guaiacum. By Mr. Willi 

 . Brande. Erom the Philosophical Transactions, IB,06. 



389 



AM 



y^MONG the numerous substances which are comprehend- Guaiacum 



ed under the name of resins, there is perhai)s no one which P.^^^'^f^*^® ^'^""y 



' ' sin2,ular prO' 



possesses so many curious properties, as that now under con- pertics. 



sidcration ; and it is remarkable that no more attention has 



been 



