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IV. — The Theory of Immersion. 



By Kev. S. Leslie Brakey; M.A. 



PaH 11. 



The essential part of the question remains, — to determine in what 

 way we are to explain the higher definition of immersion lenses ? 

 It is not to be explained by difference of angle, because in glasses 

 as hitherto constructed the angles are the same. Can it be accounted 

 for by the difference in the loss of light by all the reflexions at the 

 bounding surfaces of the media ? This opinion has been expressed 

 in a general way by Mr. Wenham, and it is easy to see at once that 

 the amount of such losses is a thing which from its nature admits 

 of exact calculation, so that we can provide ourselves theoretically 

 with a test how far the cause assigned is an adequate cause. This 

 is what I propose to do in the present paper, with the view of 

 course, ultimately, of having the results which follow from theory 

 compared with those which may be found experimentally. 



I must, however, premise that in this I am going on the as- 

 sumption that the immersion lenses do actually possess the supe- 

 riority of definition which has lately been ascribed to them. 

 Whether this assumption is justified by the facts is a question on 

 which I would rather not pronounce an opinion. I confess that I 

 have not myself made out that strong, clear, and unmistakable 

 diftereuce which so many observers profess to see ; at least, I think 

 I should not have made it out, if I had not so often been told it 

 was there. Where we are convinced beforehand by a consensus of 

 authority that such or such ought to be our perceptions, we are apt 

 to fancy we do perceive what we ought to perceive, as in judging 

 the merits of wine; and I have never been able to discriminate 

 exactly how much of the difference I saw was really due to my eye- 

 sight and how much to my belief. And perhaps a good many 

 others, if the truth were known to themselves, would find that 

 something of what they profess to see so clearly is likewise due to 

 their convictions as much as to their eyes. Nevertheless there is 

 certainly of later times so decided a balance of opinion expressed in 

 favour of these, by observers who seem to have worked carefully 

 with them, that I think it may, for the present at any rate, be 

 assumed that there is in some way and to some extent a superiority 

 actually ascertained, and which may be relied upon. 



We have to see, then, how far it may be explained by the loss 

 of light at the different surfaces, so as to get the amoimt finally 

 transmitted in all the different cases into the front of the object- 

 glass. Beyond this, of course, we have no business to follow them 

 for the purposes of comparison in this inquiry, because their sub- 



