108 THE AMERICAN MONTHLY [June, 



visibility of fine lines when examined with oblique light and a polarizing 

 prism, rotated until it partly darkens the field, used above the eye-piece. It 

 is said that the lines of A. pelhicida become much more sharply defined with 

 the prism, and even with lenses that give but indifferent resolutions without 

 the prism they become quite sharp when it is used. The explanation given 

 of this surprising result scarcelv seems to explain, for it is merely this : — 

 ' Probably the efficacy of the prism, when used with a lined test, lies in the 

 fact that it intensifies the diffraction spectra when it is placed in a certain direc- 

 tion to it,' which seems to signify that it increases the clearness of the lines 

 because it does so. 



Prof. Rogers' suggestion for a constant of nature seems exceedingly well- 

 founded. It may at first seem surprising that the expansion of a bar of metal 

 can furnish us with one of the most absolute constants we possess, but it seems 

 to be a fact ; and certainly it is the constant one most readily to be appealed 

 to under all circumstances, and most conveniently measured with precision. 



The limit of visibility of minute objects seems still to be misunderstood. 

 The matter has been occasionally discussed in these columns, and it is unfortu- 

 nate for Prof. Roscoe that he had not been a ' constant reader ' of this Journal 

 before he ventured to deliver his presidential address before the British Asso- 

 ciation last year. He placed the limit of visibility at jip^o-o of a centimeter, 

 about yuo^^-o IT of ^" inch. Probably Dr. Royston-Piggott is the only person 

 who claims to have seen i7]Tr(W(7o of ^"^ inch, but his claim seems to be well 

 founded. In any case very much smaller specs than jot/ooo h-^ve certainly been 

 seen, and it would be interesting to know the exact size of the almost invisible 

 spores that Dr. Dallinger has discovered, growing from an irresolva1)le cloud 

 into visible germs, with all the organization and potency of life. 



It is a pleasure to congratulate Mr. Wolle upon the completion of his val- 

 uable book on the fresh-water algce. Although I have not seen the finished 

 volume, its pages and illustrations will not seem unfamiliar to me, for many 

 of the latter were already drawn before I left home, and much of the manu- 

 script had been written. It is the result of long, meritorious, and enthusiastic 

 work by an earnest student, whose years number already threescore and ten. 

 I trust both the book and the author will receive the full recognition due them 

 from students of the alga' in every part of the world. The two companion 

 volumes, on Desmids and Algis, are enduring monuments to the author's con- 

 scientious industry and accuracy of observation. 



Osaka, Japan, March q, /8S8. 



The Microscope in Medicine. 



By W. D. BIDWELL, M. D. 



In the practice of medicine and surgery everything is changed since the 

 last century, and to no one factor is this change more due than to the micro- 

 scope. A few of the benefits are these : — Anatomies of the present day give 

 the histological structure of every part of the human organism, which adds 

 as much to our knowledge of how the body is constructed as did dissection 

 to those crude and even absvu'd ideas held by the early practitioners. Micro- 

 scopical anatomy naturally led to a more exact physiology — the discovery of 

 nerve fibres terminating in little plates in the skin of the finger necessarily 

 gave a clearer idea as to how" the sensation of feeling was received and trans- 

 mitted. Every organ was better understood and its function made more 

 evident by the use of the microscope. 



In pathology the various diseased structures, tumors, and fluids are daily 

 prepared and placed upon our slides. It is largely owing to the microscope 



