136 THE AMEBICAN MONTHLY [June, 



Every druggist knows that the Digitalis purpurea of our gardens 

 contains a much less proportion of digitaline than that grown in the 

 wild state. Is it not possible that the specific bacillus of a given disease 

 may, under conditions not yet understood, likewise* vary its product of 

 leucomaine? Or, to extend still further the shaky platform of possi- 

 bility, may we not in time rear a digitalis that will not contain digita- 

 line, and a Bacillus anthracis that will not in its life-history evolve 

 anthracine? 



It seems improbable that a plant consisting of a single cell is less sus- 

 ceptible to variation than is an elaborate organism like Indian corn ; 

 yet the gross differentiae between the Indian corn grown in New England 

 and that grown in Kansas are greater in degree than the differences 

 that characterize species in many genera — the genus Carex, for ex- 

 ample. 



Under changed environment, the wild boar, which produced only 

 brawn and bristles, has been transformed into the domestic hog, which 

 produces only fat. It is hard to believe that a microbe would not, under 

 equal change of conditions, show equal variation in form or product. 



An Interesting Experiment with the Microscope. 



By Prof. W. F. DURAND, 



AGRICULTURAL COLLEGE, MICH. 



The following experiment, though not, perhaps, pertaining to micros- 

 copy in the strictest sense of the term, is, however, so beautiful and so 

 simple that no one havmg a microscope should fail of trying it. 



Before describing the nature of the experiment, it may be well to 

 refer briefly to the structure of the retina of the eye. This, as is well 

 known, consists in general of an expansion of the optic nerve over the 

 back of the eye between the choroid coat and the vitreous humor. Its 

 structure is very complex, being divided by different writers into from 

 four to ten separate layers. On the anterior or inner surface, a little to 

 the outer side of the centre, is a yellow patch, within which is a well 

 marked depression. This yellovs^ patch is called the macula lutea^ and 

 the depression the fovea centralis. The in acuta lutea is the most 

 sensitive portion of the retina, and the fovea centralis is the part 

 whereon rests the image of an object directly looked at. That is, if 

 the eye directly looks at an object, its image is formed on this depres- 

 sion of the fnacula lutea. It is, therefore, that part of the retina 

 which does, so to speak, nearly all the work. The fibres of the macula 

 h^tea have the direction of ra3'S diverging in all directions from the 

 centre of \\\t. fovea centralis. One of the layers of the retina is that 

 of the rods and cones, as they are called. The rods are long cylindri- 

 cal bodies packed in closely with their axes at right angles to the plane 

 of the retina. The cones are somewhat conical or bottle-shaped bodies, 

 interspersed among the rods. In \}i\Q fovea centralis the cones alone 

 are found. These bodies — the rods and cones — are supposed to receive 

 the vibratory influence of the rays of light. This is then transmitted 

 to nerve cells connected with the filamentary expansion of the optic 

 nerve. 



Ramifying through all parts of the retina except the fovea cefttralis 

 is a capillary network of veins and arteries. This network, together 



