182 Transactions of the Society. 



its variation must either be confined to narrow limits or must be 

 compensated for by a corresponding change in the objective. Now 

 there is a great difference in regard to this requirement between 

 the objectives of low and wide aperture, in particular with the dry- 

 system. An objective of a few degrees is almost insensible, it may 

 be focussed at the bottom of a trough of water without any loss of 

 performance. With 30" difierences in the cover-glasses within the 

 usual hmits are still inappreciable, and an object may be seen at 

 the depth of a drop hanging on the under surface of a cover-glass. 

 "With 60^ a deviation of the cover-glass from its standard thickness 

 by not more than • 1 mm., or a corresponding increase of the 

 depth of the preparation above the actual focus, will introduce 

 perceptible aberrations and a visible loss of definition if not com- 

 pensated for. "With an aperture exceeding 100'^ in a dry lens the 

 same result will arise from a change of thickness of 0*02 mm. 

 only. To preserve the best correction in such a system would 

 necessitate a change of the correction-collar for almost every change 

 of focus in the inspection of successive layers, unless the prepara- 

 tion is exceedingly thin." 



There can be no doubt that the majority of the recorded 

 histology of the minuter structures will have to be worked over 

 again with carefully corrected objectives. 



Amongst the many excellent inventions for facilitating earnest, 

 and of necessity, rapid microscopical work the apparently trivial 

 arrangement perfected by Mr. E. M. Nelson, for changing objectives, 

 must commend itself to every one. It facilitates the rapid inter- 

 change of objectives without the necessity of triple or quadruple 

 nose-pieces and, in fact, will do away with the necessity of employing 

 a nose-piece at all, a proceeding greatly to be commended, for 

 those implements are often slip-shod abominations and bringers of 

 error. 



It is interesting to find that the invention utilizes a warlike 

 appliance, and that breech-loading cannon, those latest develop- 

 ments of civilization in which, according to Herbert Spencer, the 

 greatest admiration is obtained by those men who kill their 

 enemies, have been studied and copied for the purposes of a pursuit 

 which amongst others is making men more thoughtful and amenable 

 to reason. Probably the Nelson adapter will replace the screw at 

 some time or other, and it certainly cannot have some of the 

 demerits of a coarse short thread, which is bound to interfere with 

 the proper plane of the object-glass, and with the correct axial 

 path of the rays into the body of the instrument. 



In concluding these remarks on the Microscope itself I must 

 enter a protest against the clumsy method of pushing a glass slide 

 with a valuable and important object upon it with the fingers, under 

 the objective and moving it about. Cheapness of the instrument and 



