138 



THE AMERICAN MONTHLY 



[J"iy, 



shows dots — hence obHque light is mis- 

 leading. One need not look far to dis- 

 cover a fallacy in the argument thus sug- 

 gested. Apart from the fact that the true 

 structure of an object showing very min- 

 ute striae cannot be inferred from its ap- 

 pearance in a microscope, it is well 

 known that the capabihty of resolution 

 depends upon angular aperture, and it is 

 impossible to utilize angular aperture with- 

 out angular illumination. Whatever res- 

 olutions are effected by strictly central 

 light, are, nevertheless, due to angular 

 light, whether the light enters the lens 

 from an illuminator of wide angle, or 

 from the brightly illuminated object it- 

 self. 



— It is, we believe, a very general opin- 

 ion that the green color of oysters, some- 

 times observed, is due to a vegetable 

 growth, — parasitic plant. Mr. J. A. Ryder 

 has not been able to find any evidence of 

 a vegetable parasite producing this color, 

 nor does he believe that the coloring 

 matter is chlorophyl. The color seems 

 to be in the blood-cells, but whether de- 

 rived from some derangement of the 

 normal condition or not, is not known. It 

 is not unlikely that the nature of the food 

 may have some influence in producing the 

 color. 



— The latest catalogue from Mr. Carl 

 Zeiss is much larger and more fully illus- 

 trated than any of its predecessors. It 

 contains figures of the microscope-stands 

 with full descriptions, and a list of the ob- 

 jectives and their magnifying power, but 

 the arrangement has been changed, and 

 some additional information added to this 

 part. Mr. Zeiss qow makes a corrective- 

 adjustment for his homogenious immer- 

 sion lenses, but the , object of the adjust- 

 ment is to correct for the varying length 

 of tube, and not for different thicknesses 

 of cover-glagses. 



— Dr. Vincent Harris has published an 

 account of some experiments in double- 

 staining blood-corpuscles, in the Qicar- 

 terly Jotirnal of Microscopical Science. 

 Among the various combinations he has 

 used, the following seem to be the best: 

 — Rosin and iodine green. This stains 

 colored corpuscles bright red, with bluish- 

 green, nuclei. Colorless cells were of 

 three varieties, viz. those staining entirely 

 green, those becoming yellowish red with 

 green nuclei, and large masses of nu- 

 cleus-Hke bodies, said to be developing 

 colorless corpuscles, which become of a 



deep green. Fuchsin and methylen blue. 

 This combination is highly recommended ; 

 it seems to be best of all, unless fuchsin 

 and soluble blue be excepted as equally 

 good. The method of staining is this: 

 dry the corpuscles in a thin, even layer 

 on the cover-glass, then cover them with 

 the red dye, wash in water, apply the 

 green or blue dye, wash, dry, and mount 

 in balsam. 



— Mr. Washington Teasdale, of Leeds, 

 England, who has done some very fine 

 work in ruling on glass, has prepared a 

 slide for testing the illumination of a 

 spot-lens, paraboloid, and condensers 

 with stops. To get the best results of 

 dark-field illumination it is necessary to 

 have the light strike the object from all 

 sides alike, but it is not always easy to 

 know when the light is properly adjusted. 

 Mr. Teasdale uses a slide ruled with 

 bands of lines, not very closely ruled, 

 crossing at various angles. By examin- 

 ing such a plate with a spot-lens or parab- 

 oloid where the light comes in from all 

 sides the lines are all equally bright on a 

 dark ground, but when the light is un- 

 equally adjusted some of the lines are 

 dull or almost invisible. 



— Those who, from economy, or for 

 pleasure, make their own accessories, may 

 utilize a suggestion that comes from Mr. 

 Teasdale for making a spot-lens. A 

 good-sized glass fish eye, such as can be 

 obtained from dealers in naturalists' sup- 

 plies, mounted in a piece of cork, makes 

 a very satisfactory spot-lens indeed. 



— Messrs. Ross & Co., have just come 

 into possession by purchase, of all the 

 Dauget optical glass left by the death of 

 Mr. Dauget. The peculiarity of this 

 glass which makes it especially valuable 

 for optical purposes is that perfect achro- 

 matism can be obtained with a double 

 convex line of equal curvatures on both 

 sicl,es. It is also regarded as purer than 

 any other glass, because it is made in an 

 open crucible. Messrs. Ross & Co., have 

 been very fortunate in securing a large 

 quantity of this valuable material. 



— Mr. J. Grunow has improved his 

 camera hicida by a slight change in the 

 size of the opening through which the 

 image of the object on the stage is seen. 

 By reducing the opening to a diameter of 

 y^Q of an inch, he states tiiat the pencil 

 point is still more clearly seen, while suffi- 

 cient light comes frorn the object to show 

 the details. 



