THE ADMIRALTY TELEGRAPH. 271 



transmitted, and the gradual transmission which we have just mentioned. 

 Dolland's achromatic glasses, a great improvement on the telescope, 

 were made on this principle. He composed the object-glass of the 

 telescope of crown-glass and flint-glass, so that while, by the combined 

 effect of their convexities, they drew the rays of the focus, the disper- 

 sive power of the one was counteracted by that of the other. Let us 

 endeavour to explain this. A beam of light, being composed of the 

 different coloured rays, passes through a prism. Instead of passing 

 onward in a straight line, it is refracted in distinct, and, consequently, 

 colored rays. Whilst the whole of them are bent or refracted at an 

 angle, they are also diverging from one another. Their deviation from 

 the straight line is their refraction: their diverging from each other is 

 their dispersion. These properties being distinct, it is conceivable 

 that glass of a different chemical composition may affect the one to a 

 greater degree than the other, and, therefore, that a lens may be com- 

 posed of different kinds of glass (crown-glass and flint-glass, for example), 

 so that the convergence of the rays into a focus may be obtained with- 

 out the dispersion of the rays, and the consequent production of false 

 colours round the image. This is what Dolland nearly accomplished, 

 and upon these principles. That the effect of this very artificial arrange- 

 ment is attained in the eye is a remarkable proof of the perfection of its 

 adaption to the properties of light. The last circumstance which we 

 may mention in continuing the comparison, is the drawing out of the 

 tube in the telescope to accommodate the foci of the glasses to the distance 

 of the object. It is sufficent to say, that the eye possesses this property 

 of accommodation. That we do not understand how the operation is 

 performed, only strengthens the argument in favour of the perfection of 

 the eye: since the power exists, and is exercised with an ease which 

 hardly permits us to be sensible of it. 



THE ADMIRALTY TELEGRAPH 



Is the invention (or rather the improvement of the French semaphore, of 

 M. Chappe) by the late Sir Home Popham; it .consists of an upright post 

 with two indicators, which move upon centres one above another; the 

 mast is made to turn round upon its axis, so as to present its arm 

 successively to all quarters as may be required. The movements are 

 very simple; they are effected by iron spindles and endless screws, so 

 that the indices below are certain to accompany the indicators 



