Hagen.] 358 [March 10, 



progress is arrested. The increase of the angular aperture increases 

 the two aberrations to be corrected, and materially weakens the pen- 

 etrating power. Judging from an examination of the test plate of 

 !Nobert, it would appear that the best instruments of any countiy 

 differ but little in power. It was stated, in a recent meeting, that 

 Messrs. Stodder and Greenleaf had resolved the highest groups, a 

 thing never before accomplished with any instrument. This statement, 

 however, is doubted by their learned countryman, Dr. Woodward. 



The test plate of Nobert, dividing the inch into more than one 

 hundred and twelve thousand parts, is generally adopted as a very 

 good test object. But even here a very important consideration in 

 forming a thorough and correct judgment exists, and is almost con- 

 stantly overlooked; I mean the difference in the aberration of the 

 eyes of the observers. There is no doubt that different observex's 

 obtain different results from the same instrument; of course a greater 

 dissimilarity arises in the use of the same test object with separate 

 microscopes. All attempts to coiTCCt this personal aberration are 

 still unreliable and unsatisfactory; therefore the microscopic, photo- 

 graphs which are brought to so admirable a degree of perfection, are, 

 in fact, the surest test objects now existing for the power of an 

 instrument. 



Besides this personal difference there exists a very considerable 

 one resulting from the continual use by each observer of one particu- 

 lar instrument. In this connection I recall the striking fact, that as 

 the celebrated microscope of Leeuwenhoek arrived at the Royal So- 

 ciety in London after his death, no one was able to see the objects 

 observed and described by him. An experienced observer will often 

 see much better with his own imperfect instrument, to which he ia 

 accustomed, than another person would do with a far superior micro- 

 scope. 



Doubtless the most important matter for microscopical science is 

 the jirice for which the instrument can be obtained. The cheaper the 

 instrument the larger the number of observers. In Europe, ten 

 years ago, about two thousand large instruments were manufactured 

 every year; now the number is more than double. Surely for a 

 physician, and for many other observers, an amplification of moderate 

 size, from two hundred and fifty to three hundred diameters is suffi- 

 cient. Prof. Ehrenberg, in Berlin, — and I believe no living observer 

 has made so much use of the microscope, uses almost constantly in 

 his work an amplification of three hundred and fifty, and in some 



