34 Transactions of the Society. 



On the Limit to Defining-jpower, in Vision with the Unassisted 

 Eye, the Telescope, and the 31ic7Vscopc. 



By J. J. LiSTEE. 



(Dated 1842-3.) 



The course of observations now offered was undertaken about 

 eleven years ago, i.e, in 1831-2, when some main causes of in- 

 distinctness in the Microscope having been recently surmounted, I 

 was led to enquire, whether it offered a prospect of such progressive 

 improvement as might admit of carrying researches into the depths 

 of creation to an indefinite extent ; or whether, as some appearances 

 made me suspect, a limit had been already approached that was 

 impassable by man. 



The books within my reach, which contained the observations 

 on vision of Newton, Jurin, Sir W. Herschel, and Young, with 

 some others, gave no answer to the question, though they suggested 

 to me experiments that promised to furnish one. 



The results of these I should have communicated earlier, 

 but on perusing Sir J. Herschell's article on " Light " in the 

 Encyclopaedia Metropolitana, then recently published, I found some 

 of the beautiful experiments of Fraunhofer and his own bore such 

 analogy to mine, that I was apprehensive the general law of dis- 

 tinctness which I had found to prevail might be considered as 

 deducible from them, and therefore contented myself with mention- 

 ing it to a few of my friends. 



However this might have been, the subject has since, I believe, re- 

 ceived no further elucidation, while the extended use of the Micro- 

 scope of late, and its application to objects that tax its highest 

 powers, make it important, both for the maker and the observer, 

 that the limit and character of those powers should be known. 



It is mainly this consideration that has induced me now to 

 review and make some additions to my experiments of that time, 

 although they relate equally to vision with the naked eye and the 

 telescope as with the Microscope, and I am willing to hope that 

 with reference to all, they may yet be found not without utility. 



The causes that limit defining-power in vision may be divided 

 into those proceeding from the nature of light, and those which 

 depend on the constitution of the eye, and their effects are not 

 unfrequently perceptibly combined. This may be instanced by 

 two of my early experiments, in which the tendency of an enlarge- 

 ment of the optic pencil to increase distinctness was modified by 

 the sensibility of the retina, although the brightness of the object 

 was not such as to produce uneasiness to the eye. 



1. For the purpose of ascertaining the effect of pencils of 

 different sizes received direct from objects by the naked eye, small 



