NEW BOOKS, WITH SHOKT NOTICES. 129 



for wliile it has involved immense labour upon the author, it tends to 

 prevent us from assuming as a certainty that cholera is due to the fungus 

 described by Hallier ; and that cannot be held in the History of Science 

 to be a matter of no importance. Dr. Levs^is, we believe, proposes to 

 continue his researches, and we hope he may, for good as his present 

 work is, we may even expect a better labour in the future. 



" On a Searclier for Aplanatic Images applied to Microscopes, and 

 its Effects in increasimj Poiver and improvinq Definition. By G. W. 

 Eoyston-Pigott, M.A., M.D., Cantab., M.E.C.P.," F.C.P.S., F.E.A.S., 

 formerly Fellow of St. Peter's College, Cambridge. Communicated 

 by Pi'ofessor Stokes, Sec. K.S." — A paj)er on a new instrument having 

 recently appeared in the ' Philosophical Transactions,' and been re- 

 printed from them for private circulation, we think some notice of it 

 will not be unwelcome to our Fellows. 



The writer seems to have long felt, with other microscopists, that 

 something remained to be done — first, in overcoming the great incon- 

 venience attending the use of very deep eye-pieces, deep objectives 

 dangerously close to the covered object ; and next, in adding further 

 corrections, in addition to those of .the screw-collar for diiferent thick- 

 nesses of glass cover. In these j)rincij)al points he appears to have 

 succeeded. 



The traverse of a system of adjustable lenses between the fixed 

 eye-piece and objective produces some remarkable changes in the 

 definition, better or worse, according to the skill of the operator, and 

 admitting a much wider range- of chromatic and spherical correc- 

 tions than can possibly be obtained by merely separating the objective 

 lenses by means of the screw-collar. The author of this paper states 

 (p. 598) :— 



" Using additional compensating lenses to gain increase of power, 

 intei'mediately placed between eye-piece and objective, the finest defi- 

 nition is obtained when each of the three sets, viz. lenses, observing 

 and image-objective, are similarly, though slightly over-corrected, as 

 comj)ared with a standard defining distance of 9 inches.* 



" Although a fine definition seemed now attainable by means of 

 supplementary compensating lenses, if judiciously introducing ba- 



* " It is convenient to define the aberration to be positive or negative, or the 

 lens to be ovei- or U7ider corrected, by the simple fact that an ordinary lens causes 

 the excentrical rays to cross the axis at a point nearer the centre of the lens than 

 the centrical rays, in wliich case, and in all analogous cases, it may be said that 

 the lens is under-corrected and afflicted with a negative aberration. All the best 

 objectives are now constructed on the principle of having the posterior sets over- 

 corrected and the anterior under-corrected so skilfully as fo destroy by opposite 

 errors nearly the residuary aberration; but the opinion may be hazarded that 

 futiu'e combinations will yet be found which will completely throw into the shade 

 the present powers of the microscope, when, perhaiDs, we shall be in a better 

 position to attempt to determine the microscopical features of molecular life, at 

 present probably beyond its grasp, as no single particle so small as the tj-ottoo*^ 

 of an inch in diameter can be clearly defined, if isolated, until residuary error is 

 very much more reduced. 



" It is to be regretted that the precise nature of the marvellous combinations 

 invented by Professor Amici for objectives remains imknown. As one of the 

 Jurors in the Paris Exposition, his microscope necessarily remained both uncele- 

 brated and unelucidated in the Keports. 



