128 THE MICROSCOPE. 



announcing Prof. Rogers, of Cambridge, who read a very entertain- 

 ing paper upon micrometric measurements, showing some exceed- 

 ingly fine apparatus and delicately ruled steel and glass bars, in 

 illustration of his subject. 



He demonstrated the irrelevancy of all " statutory " units of 

 measure, pointing out the futility of the prevailing methods of guard- 

 ing against fluctuations caused by changes in temperature. He 

 showed the fallacy of such a term as " probable limit of error," 

 and the occurrence of certain errors overlooked in making correct 

 measurements, instancing and dwelling upon the focal error. He 

 exhibited a combined yard and metre, which was ruled to the utmost 

 degree of accuracy possible to be obtained by any contrivance or 

 surroundings known to science. At the conclusion of Prof. Rogers' 

 remarks, the society adjourned to 8 o'clock in the evening, at which 

 hour the fine lecture room was completely filled. Four new names 

 were added to the roll. — Geo. C. Taylor, of Tibadeaux, La., better 

 known as "Old Gray Beard," exhibited a new mechanical lamp — a 

 modification of the Hitchcock lamp, in which the burner is brought 

 very low upon the table, while the intensity of the light is regulated 

 by a movable diaphragm, which increases or curtails the volume of 

 air admitted to the fan. A practical test of the light, in resolving 

 fine lines, proved its superiority over all lamps yet devised. W. H. 

 Walmsley, your valuable contributor from Philadelphia, was on hand 

 with several new appliances to be used in microscopy and in photog- 

 raphy. He gave us a practical paper on microphotography, illus- 

 trating the same by taking a microphotograph of a fly's tongue, on 

 the spot. This brought out a very useful discussion from the photo- 

 graphic fraternity, in which numerous suggestions were made, and 

 from which many valuable hints were gleaned, too numerous, by far, 

 to enumerate in this letter. Prof. Hamilton L. Smith then devoted 

 the remaining hour-and-a-half to his memoir of Charles Spencer, 

 which you will probably print and so spare me in attempting even a 

 synopsis of it. 



The Thursday morning session opened at nine o'clock, when 

 four more members were accepted, and several reports of commit- 

 tees received and acted upon. Prof. McCalla, from the committee 

 upon the publication of a quarterly journal, reported that it was not 

 just then advisable for the society to establish a separate and inde- 

 pendent journal. — Uncle Griffith exhibited his improved club stand. 



