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VI. — Object-glasses and their Definition. 



By F. H. Wenham, Vice-President E.M.S. 



I MAY offer some excuse or apology for the controversy under the 

 above head with Dr. Pigott. Why he should state that he has 

 incurred my displeasure on the items recapitulated in the last 

 Journal I am at a loss to imagine, as I have not been out of 

 temper, or had any feeling of the kind. At the outset he made 

 some statements concerning object-glasses, which appeared to me 

 questionable, and fairly threw down the gauntlet by saying that 

 " the battle of the glasses will have to be fought." All these 

 questions must be resolved in a practical sense at last. It could 

 not be expected that any of the professional makers would express 

 opinions, or divulge their practice ; and seeing grounds for objection, 

 I accepted the challenge, and during the controversy, or " battle," in 

 the usual course I bring forward any facts that will tend to prove the 

 error. With every desire to arrive at the truth this is oftentimes a 

 source of irritation. Events have shown that the first points need 

 not have been dwelt upon. I might well have left the beaded ap- 

 pearance of the Poclura to the decision of others. I have lately 

 looked over a great number of slides, and now have particular spe- 

 cimens which easily develop beads with a ^ inch. Traced to its 

 source with the highest powers, this is evidently caused by inter- 

 costal corrugations of the membrane. I therefore endorse the opinion 

 of the most diligent and accurate observer of this class of objects, 

 that the beads are " ghost beads," and the ! ! ! spines are not " spuri- 

 ous" as Dr. Pigott states. We are all working with the same 

 view of obtaining results, and must admire the industry with which 

 Dr. Pigott has been elucidating the primary laws of refraction and 

 reflexion, by his papers in the Journal and ' Transactions ' of the 

 Society during the past year ; and though the caustic curves, and 

 figures of aberration from a variety of concave and convex reflecting 

 surfaces, are to be found in Potter's and other treatises on Optics, 

 still the rehearsals may be welcome to those who do not habitually 

 refer to these works. 



With respect to the immersion lens (the true advantage of 

 which I have never disputed),* I must state that Figs. 2 and 3, 

 page 23, Jan. 1871, are not mere imaginative diagrams arranged to 

 meet a far-stretched theory, but transcribe a positive fact. All the 

 distances there shown, both for thickness, radius, diameter, and focal 

 distance, were taken from actual and repeatedly-verified measure- 

 ments, when adjusted for the same object and cover. The aperture 



* It might be inferred from the tone of Dr. Pigott's communications, that he 

 defends the immersion principle against a number of opponents. — "Where are 

 they? 



