320 Velocity of the Rays which proceed from the Sun, 



Before concluding it may shortly be hinted, that the prin- 

 ciples which have now been explained may possibly serve 

 also for discovering whether the colour-making rays them- 

 selves which constitute the solar white light be all of them 

 emitted with the same or with a different velocity. 



If ever it shall be found practicable to procure a very 

 sharp image of the sun by a speculum of such small aper- 

 ture and feeble reflecting power as to admit of our viewing 

 the image in the way of Dr. Herschers front view, at a 

 local distance of about a hundred feet, and without the in- 

 tervention of any medium put before the eye-glass, it would 

 be. well worth while to examine carefully whether the ex-*- 

 treme verge of the limb of that image on the opposite sides, 

 as pointed out by the directrix so often mentioned, assumed, 

 in any degree, tints allied to the red- and violet -making 

 rays. If so, this would manifestly be the effect of aberra- 

 tion arising from the different velocity of the extreme pris- 

 matic light. Though this were so small as to make the 

 difference of aberration half a second only, still the space 

 at tiie image corresponding to this angle would be three of 

 those parts of which an inch contains a thousand. Such 

 therefore might be considered as the greatest breadth of the; 

 incipient crescents, so to speak, in this instance. These, 

 if at all perceivable, would be distinguished from one an- 

 other by the outermost confines of the one being related, in 

 point of tint or colour, to the least refrangible rays, and of 

 the other to the most refrangible, in looking; for such a 

 criterion, our sight would enable us to discern very minute 

 and delicate differences. 



It is not at present asserted that a telescope such as ha& 

 been described would have power enough to decide the me- 

 rits of that theory which makes the different refrangibility 

 of the prismatic ravs of light to depend on their different 

 velocity before incidence. But should the power be suffi- 

 cient, and supposing the experiment practicable, if no sym- 

 ptoms of such incipient crescents were perceivable, the ne- 

 gative of the theory would thereby be demonstrated. Ip 

 cither case, therefore, such an experiment would be highly 

 valuable also by settling a matter which has long been merely 

 conjectural, and which, besides, is intimately connected 

 with other elementary points of great importance to optical 

 science, themselves doubtful, at this day, by resting on 

 hypotheses. 



LIU. Ac- 



