TRANSACTIONS OF THE SECTIONS. 375 



Thus, when from any cause the rays do not converge accu- 

 rately upon the retina, the dispersion is sensible. 



To some eyes, however, it is scarcely appreciable, and in all 

 cases is but small. 



So long as direct centrical pencils have their mean rays 

 converging accurately upon the retina, no colours are percepti- 

 ble. 



When the incident rays are partially intercepted by an 

 opake body, the image is formed by eccentrical pencils, and 

 the colours become very conspicuous. 



Fraunhofer admitted the different prismatic rays successively 

 into a telescope, and found it necessary, in passing from the 

 red to the violet ray, to adjust both the eye-glass to the object- 

 glass, and the eye-glass to the micrometer-wire, in order to see 

 the wire distinctly in the different sorts of light. 



The whole displacement must be the sum of the chromatic 

 aberrations of the object-glass, the eye-glass, and the eye. 



Fraunhofer does not notice the first ; says that the second 

 is allowed for; and takes the residuum as the dispersion of 

 the eye. 



He elsewhere states that the telescope was not perfectly 

 achromatic; and as the data are not stated, we cannot regard 

 the inference as conclusive, and Fraunhofer admits that it is 

 not precise. 



The author of this paper has tried similar experiments, but 

 found the displacement so small that he is quite in doubt whe- 

 ther any was requisite : and considering that the aberrations 

 of the lenses may be uncertain to a larger amount than the 

 quantity sought, it cannot be satisfactorily deduced by this 

 method. 



The theories which have been proposed are by no means 

 satisfactory. 



D'Alembert conceives that the agitation {^branlemeni) occa- 

 sioned on any one point of the retina, extends itself to the 

 neighbouring points, and thus each point is influenced by the 

 sum of the effects due to all the coloured rays at once, and 

 simple vision results. 



Euler's explanation is grounded on hypothetical assumptions 

 as to the dispersive powers. 



This is remarked by Dr. Maskelyne, who by calculation 

 finds the aberration so small as to be insensible. He, however, 

 assumes the dispersions only by analogy from the proportions 

 assigned by Newton. 



Dr. Wells leaves it as a point yet to be explained. 



Mr. Coddington, admitting a considerable difficulty in the 



