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VI. — 0)1 the Investigation of Microscopic Forms hy means of the 

 Images ivhich they furnish of External Objects, with some 

 Practical Applications* By Prof. 0. N. Eood, of Troy, N.Y. 



[We have been requested by Professor Smith to publish the 

 following letter, which, notwithstanding its date, we do with great 

 pleasure, as it is of especial interest at the present moment. — 

 Ed. ' M. M. J.'] 



It would hardly occur to a physicist, who was requested to deter- 

 mine whether a certain disk of glass was a convex or a concave 

 lens of slight curvature, to attempt a solution of the question by 

 glancing along the two sides ; on the other hand, neglecting even 

 to look at the glass, he would at once bestow his undivided atten- 

 tion upon the images of external objects formed by it, and thus 

 with ease and certainty decide upon the nature, degree, and regu- 

 larity of its curvature. 



The simple idea here enunciated seems hardly to have been 

 applied to the study of microscopic forms, though from some ex- 

 periments lately made in this direction, I am firmly convinced that 

 this method of determination is destined hereafter to play a most 

 important part in microscopic observation. To the microscopist it 

 Avill prove as powerful a means of investigation as it now is in the 

 hands of the optician. 



The most convenient and efiective mode of proceeding in this 

 case which has occurred to me is the following : the microscope is 

 brought into a horizontal position, the mirror removed, and the 

 illumination supplied by a candle or lamp placed in the axis of 

 the compound body at a distance of not more than 3 inches from 

 the stage. If now a small sphere of glass be properly supported 

 on the stage, it forms behind itself a very minute inverted image of 

 the flame of the candle ; uj)on drawing back the compound body 

 slightly, this image comes into focus, and is seen of course in an 

 erect position. When a rod of yoth of an inch in thickness is 

 moved up and down between the flame and the globule, an image 

 of it is seen in the microscope with great distinctness, and it is 

 observed that the motion of this image follows in all respects the 

 motion of the hand. Upon replacing the sphere by a minute con- 

 cave lens, as an air-bubble in water, the reverse takes place ; to 

 gain distinct vision of the flame it becomes necessary to move the 

 compound body within the focus, the image of the flame is seen to 

 be inverted, and what is practically more important, the motion of 

 the rod seems reversed. It will happen very generally in applying 



* See some remarks by Mr. Ckirles Stewart, F.L.S., in vol. viii., p. 281, of 

 ' M. M. J.' 



