Royal Microscopical Society. 251 



Unfortunately, the information regarding the structure of the 

 optical part of the instrument is of such a conservative character, 

 both in the original paper, published in the ' Philosophical Trans- 

 actions,' and in the ' Journal of the Royal Microscopical Society,' 

 that it does not furnish data sufficient for its perfect construction ; 

 and as it appears to have resulted from " error and trial " rather 

 than from computation, hence one reason why others may not be as 

 successful as he appears to have been. I can well understand the 

 tediousness of the research and the patience necessitated to bring 

 such a method to so successful a phase, without losing defini- 

 tion. Doubtless it will stimulate both opticians and amateurs to 

 renewed efforts to cover the " residuary aberration " ; but if we do 

 not also get rid of the secondary spectrum, there is something more 

 to be achieved than is yet accomphshed. Possibly some refracting 

 cement, as Mr. Grove's purified resin and castor-oil, or Canada 

 balsam and castor-oil, if made one of the components of an achro- 

 matic "concave" or "corrector," may furnish some help. 



By no means do I wish to detract from the advantages of the 

 " aplanatic searcher " in other hands, but in mine it did not fulfil 

 the expectations entertained. It certainly furnished great facihty 

 for bringing out colour in lined objects of difierent refractive power, 

 and thus j^erhaps it may have a wide claim to our favour, provided 

 we are not carried away by the influence, by reason, as IMr. Mclntu-e 

 so aptly expresses it in his note of Jan, 20, which I take the liberty 

 to quote, that " the colom^ invests some objects with a new charm." 

 It was not found to increase the defining power of the objective, but 

 this may have been due to want of ability on my part. 



It seemed questionable whether three " over-corrected " bodies, 

 compound lenses, eye-pieces, searcher, and objective,* and the addi- 

 tional over-correction furnished by the thin covering glass, would 

 provide, by one traversing between the two others, perfect achro- 

 matism. 



It may perhaps be as well to point out the error in the original 

 paper, ' Phil. Trans.,' and the appended note in p. 129, where, in the 

 definition of the aberrations, positive and negative aberration have 

 been confounded. The same error is repeated in the present 

 ' Quart. Journal of Micros. Science,' evidently having escaped the 

 notice of the author. 



Whatever may be its value as regards depth of focus, kc, of this 

 I feel confident, that at the same time that it furnishes a very ready 

 means for increased amplification, so it also facilitates optical errors 

 in observations. The limitation as relates to light, the length of 

 tube needed, though inconvenient, are quite, if not more than, 

 balanced by the erect image and the use of low powers and eye- 



* Vide editorial remarks on Dr. Pigott's paper in the ' Phil. Trans.,' in the 

 ' M. M. J.,' No. XXVII., p. 129. 



