1883.] 



MICROSCOPICAL JOURNAL. 



185 



side is then cemented with balsam on 

 a polished glass slip, several such 

 prepared glasses being cemented side 

 by side on the same slip, presenting 

 the appearance shown in figure 34 

 (natural size), fig. 35 being one of 

 the circles enlarged. 



A perfectly corrected objective, 

 tested with the test object, and by the 

 mode of illumination above described, 

 ought to show over the middle of the 

 field a clearly defined image of the 

 groups of lines under examination 

 without any alteration of focus, and 

 the colored borders of the separate 

 partial images should not show any 

 other tints than a very narrow edging 

 of pure green, rose, or violet of the 

 secondary colors of a spectrum. 

 Spherical aberration is revealed, when, 

 with the best focussing, the clear lines 

 appear as if immersed in the middle 

 of a broader foggy streak, or when two 

 images, more or less overlapping each 

 other, merge on altering the focus, 

 into one image, somewhat broader and 

 more misty. 



A short and ready method of testing 

 approximately any objective is recom- 

 mended by Professor Abbe', as it is 

 applicable to all instruments without 

 requiring any apparatus except the 

 test object already described. This 

 may be briefly explained as follows : — 



First focus tlae test plate with 

 central illuminating rays ; then with- 

 draw the eye-piece, and turn aside the 

 mirror so as to give the utmost 

 obliquity of illumination which the 

 objective under trial will admit of. 

 This will be best determined by look- 

 ing down the tube of the microscope 

 whilst moving the mirror, and observ- 

 ing when the elliptic image of light re- 

 flected from it reaches the peripheral 

 edge of the field. As soon as this is 

 done, replace the eye-piece and ex- 

 amine afresh the object plate without 

 altering the focus. If the objective 

 be perfectly corrected, the groups of 

 lines will be seen with as sharply 

 defined edges as before, and the 

 colors of the edges must, as before, 

 appear only as those of the secondary 



spectrum in narrow and pure outline. 

 Defective correction is revealed when 

 this sharp definition fails, and the 

 lines appear misty and overspread 

 with color, or when an alteration of 

 focus is necessary to get better defini- 

 tion, and colors confuse the images. 



A test image of this kind at once 

 lays bare in all particulars the whole 

 state of correction of the microscope ; 

 it being of course assumed that the 

 observer knows how to observe and 

 what to look for. 



With the aid which theory offers to 

 the diagnosis of the various aberra- 

 tions, a comparison of the colored 

 borders of the separate partial images, 

 and an examination of their lateral 

 separation and their differences of 

 level, as well in the middle as in the 

 peripheral zones of the entire field, 

 suffices for an accurate definition of 

 the nature and amount of the several 

 errors of correction, each of them 

 appearing in its own primary form. 

 Therewith we also see that which 

 arises from aberration, properly so 

 called (faults of focussing function), 

 clearly separated from such imperfec- 

 tions or anomalies as spring from 

 mere differences of amplification be- 

 tween unequally refracted rays ; and 

 moreover we eliminate completely all 

 influence of the ocular on the quality 

 of the image. 



The American Society of 

 Microscopists. 



Continued from page 1^4. 



The space at command does not 

 warrant an analysis and abstract of 

 all the papers read. The greater part 

 are mentioned below. 



Dr. F. M. Hamlin read the first 

 paper, title, " The Microscopical Ex- 

 amination of Seminal Stains." He 

 had found by experiments upon stains 

 known to be seminal, that the method 

 of Dr. Koblanck was not satisfactory. 

 This method, with its soaking and 

 manipulation, tends to destroy so 

 many of the spermatozoa as to lessen 



