142 PROCEEDINGS OF THE AMERICAN ACADEMY 



Pleurosigma angidata with a one-inch objective of his make. Now 

 nothin'T but the enormously wide angle and the remarkably flat field 

 which he has introduced in such a low power, could enable one to 

 solve such a finely marked Diatom. Only a few years ago this little 

 unicellular plant was a test object for the highest powers of the best 

 microscopes. 



" But if this image, or the image of any minute body, is to be mag- 

 nified to any extent which may be required, by the use of the higher 

 eye-pieces, the latter must be most exquisitely corrected, as regards 

 their spherical and chromatic aberration, or else everything comes to 

 the eye in a distorted state. On this account the Huyghenian ocular 

 is not fit to be used, since it lacks just what we need here. I have for 

 several years past asserted that the next step in the increase of the 

 magnifying powers of the microscope would be accomplished by the 

 construction of a new form of eye-piece, which would augment the 

 image formed by the objective to an almost unlimited extent. At last 

 I am happy to find my prediction verified, in the most practical man- 

 ner, by the 'orthoscopic ocular' invented by Spencer. With such 

 a range of powers, then, there is hardly any body of moderate trans- 

 parency but what may be minutely investigated to its very core. 

 If a subject is too thick for the short working distance of the higher 

 powers, a lower objective may be used, and the higher oculars 

 applied to make up the deficiency. Of course I do not mean to 

 say that a certain amplification obtained by a low objective and a 

 high orthoscopic ocular is fully as good as the same afforded by a 

 higher objective ; but in case the latter cannot reach a certain internal 

 structure, the former can be used, with very little appreciable differ- 

 ence, and is by far better than the usual methods employed in such 

 cases, such as pressure or dissections and the isolation of the organ to 

 be investigated. 



" I have not had an opportunity to make frequent use of the ' ortho- 

 scopic eye-piece ' ; but Mr. Spencer has furnished me with another 

 form of ocular, the ' solid eye-piece,' invented by his pupil, Mr. Tolls. 

 This, Mr. Spencer tells me, so closely approaches the ' orthoscopic eye- 

 piece ' in quaUty, that none but a very expei'ienced eye could detect 

 the diffei'ence, and the former excels the latter in the admission of 

 light, because it has fewer reflecting surfaces. TVith this ocular and a 

 quarter-inch objective I have run the magnifying power up to two 



