VI PREFACE. 



lematical Bacillus-organism. Having the misfortune to differ in opinion 

 on this point from certain American Microscopists, who are distinguished 

 by their expertness in the resolution of lined tests by Objectives of the 

 largest angular aperture, and who enthusiastically advocate the use of 

 such Objectives as the only powers to be trusted for Biological research, 

 he has requested his friend, Mr.' Dallinger (than whom there can be no 

 higher authority on such a question), to give him the benefit of his ex- 

 perience thereon. And he is authorized by Mr. Dallinger to express his 

 entire concurrence in the opinion uniformly upheld by the Author, that 

 great ' resolving power' is only exceptionally needed in the most difficult 

 Biological investigations; what is especially required for the study of liv- 

 ing and moving organisms being such crisp and clear definition, good 

 working distance, and considerable focal depth, as high-power Objectives 

 of the widest aperture cannot afford. These qualities are so admirably 

 combined in the i dry ' l-35th of ' moderate angle ' constructed to Mr. Dal- 

 linger's order by Messrs. Powell and Lealand, that he has been able to do 

 work (of the kind just specified) with this Objective, which it would have 

 been simply impossible for him to do with the oil-immersion l-25th of the 

 same makers, although this far surpasses their l-35th in ' resolving' 

 p ower . When Prof. J. Edwards Smith, and those who side with him, 

 shall have produced Biological work of anything like the same nature 

 and quality as that of Mr. Dallinger, it will be interesting to know the 

 results of their more extended experience. 



On another point of great practical importance, the Author has 

 thought it worth while to avail himself of Mr. Dallinger's unrivalled 

 experience, the utility of ' deep eye-piecing.' For he has seen with 

 .astonishment that the enthusiastic American advocates of the widest 

 .angles for Objectives of moderate power, are claiming for such objectives 

 the advantage that they may be worked-up to any amount of amplifica- 

 tion by sufficiently ' deep eye-piecing;' solid eye-pieces of half or even 

 .a quarter of an inch being now spoken-of as in ordinary use. He does 

 not for a moment doubt that difficult lined tests may be thus shown; but 

 that it is far less trying to the vision, when exercised in continuous ivork, 

 to gain the needed amplification by a high Objective and shallow Eye- 

 piece, than by a low Objective (however wide its angle) and deep Eye- 

 piece, experience long ago satisfied him. Not having thus exercised his 

 eyes, however, upon objects requiring the high amplifications used by Mr. 

 Dallinger, he was fully prepared to submit his own judgment on this 

 question to that of a gentleman who has so well earned his title to pro- 

 nounce an authoritative verdict upon it; but, so far from having in the 

 least to give way, the Author finds himself supported by Mr. D. in the 

 most emphatic way. For he learns, not only that Mr. D.'s experience in the 

 study of the most difficult Biological objects satisfies him of the immense 

 superiority of the highest Objective that admits of good working distance, 

 combined with a low Eye-piece, over the ' strained amplification ' given 



