1892.] NEW-YORK MICROSCOPICAL SOCIETY. 25 



methods of illumination were devised which enabled the observer 

 to throw beams of light of different sorts upon the object at 

 various angles of incidence. By this means it was discovered 

 that lined shadows were cast upon the surface of valves which 

 had previously appeared entirely smooth and clear. It was 

 found that these shadows are caused by shallow furrows running, 

 -not only lengthwise of the valve, but also at right angles to the 

 median line or even obliquely to it. For a long time it was 

 regarded as the acme of manipulative skill to display this simple 

 system of striation upon the larger and coarser forms upon which 

 the distance between the lines ranges from the 20,000th to the 

 50,000th of an inch. By slow degrees the microscopist attained 

 to the ability to show at once two or more systems of lines, pro- 

 ducing a cross-hatching with square, lozenge, or hexagonal inter- 

 spaces, and at about the same time it began to be possible to dis- 

 cern upon some of the smaller specimens a striation having only 

 the 8o,oooth or the 90,000th of an inch between lines. This was 

 the maximum of attainment about thirty years ago. Since then 

 progress has been slow in this department of microscopy and each 

 small step achieved has caused a disproportionate amount of 

 labor and discussion. But after a while an advance was made 

 to the resolution of lines less than the ioo,oooth of an inch apart, 

 upon such fine species as Frustulia saxotnca and Amphipleura 

 pellucida, and the methods which rendered this progress possible 

 brought double systems of lines to view on those diatoms which had 

 before shown only one system (like Surrirella gemma), and raised 

 to the rank of well-defined dots the interspaces in the previous 

 cross-hatching upon the more robust species, such as Fleuro- 

 sigma angulatum. Then it was that microscopists ventured on 

 the important generalization that the typical form of marking, 

 throughout the whole subclass of diatomaceae, is a series of dots, 

 oftenest arranged in formal rows, but sometimes scattered irregu- 

 larly over the shell. 



As to the precise nature of these dots, there has always been, 

 and is now, a wide difference of opinion, although I venture 

 to think there is no scientific puzzle to the solution of which 

 more intelligent effort has been devoted or over which a more 

 earnest contention has prevailed. The combatants have arrayed 

 themselves in three armies, defending respectively the theory of 



