230 Ojptical Curiosities of Literature. 



trie, the field equably lighted ; but this is not altogether so. It is 

 so, however, to a greater degree than in any other illumination ; 

 and I have used it for the Binocular with an ith of large aperture 

 with perfect stereoscopic effect. This, however, is not at all times 

 very easy, the direction of the light sometimes making itself very 

 evident ; and I confess that I cannot as yet always control its action 

 80 as to reproduce effects before obtained. Moreover, in applying 

 it to very fine objects, like Podura scales, where there is in parts 

 almost, though not absolutely, contact with the glass, difficulties 

 begin to appear, of theory too as well as of practice. The limits of 

 the dissipation of the inner light have to be considered ; and indica- 

 tions show themselves of conditions hmiting the absolute inflexibility 

 of the law of total reflexion, which require for their elucidation more 

 knowledge of the laws of light than any student of optics, I fear, 

 as yet possesses. 



The other action of this lens I mention only to exclude it. It 

 works with media only ; the upper surface of the cover then reflect- 

 ing down the light. It is not therefore new in its nature, but only 

 an opaque illumination by a special method. It is also, to my 

 experience, very hard to manage ; and I do not expect much can 

 be got from it. At any rate it has nothing to do with the diagram 

 given above, or the remarks I have made upon it. 



The preceding paper was written under the impression that the 

 discussion in which Mr. Tolles was concerned had ended. From 

 the March number of the Journal, received this morning, I find 

 that this supposition was premature. Not dismayed by his two 

 fells, he puts in yet another appearance, or rather two appearances. 

 If Mr. Tolles has any enemies, which I hope he has not, they must 

 rejoice that their enemy has written, not indeed a book, but four 

 scientific papers. In these last two he has, as reviewers say of the 

 latest work of a novehst, surpassed himself. It is in truth scarcely 

 possible to discuss seriously the absurdities of these articles. That 

 Mr. Tolles meant them seriously, there can, I think, be no doubt ; 

 but if he had meant them as a jest, a more prosperous one was 

 never sent out. Changing his front once more, he thinks he now 

 really at last has caught his aperture. In his own language he 

 has designs, two at least, for its " procurement." The back com- 

 binations having failed him, he now has remedied the defect (on 

 paper). His objects he will mount in the insides of httle glass balls or 

 pillulie, which shall be quite separate from the object-glass. Then 

 when the rays come out as before at every angle, he will look into 

 these httle balls with a complete object-glass, a glass with its own 

 front, a glass of " three systems " — and so the thing is, as he says, 

 " ended." We shall see. Suppose — it is an impossibility, a twofold 



