378 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



The Limit of Accuracy of Measurement with the Microscope. 



Professor W. A, Rogers, of Cambridge, presented a paper 

 at the meeting of the National Microscopical Congress at In- 

 dianapolis, on the Limit of Accuracy of Measurement with 

 the Microscope. He finds that two experienced observers 

 can measure the distance between two lines, and obtain fig- 

 ures agreeing within such narrow limits that they are almost 

 identical. The following conclusions are drawn by Professor 

 Rogers: 1. Two equally skilful observers can measure the 

 same space within about -^-oVo of an inch, if the space does 

 not exceed -$, of an inch ; for a space of T ^ of an inch, the 

 deviation will probably amount to -yVo of an inch, in case 

 the measurements are made with a filar micrometer. 2. 

 The average deviation for accumulated errors, under simi- 

 lar conditions, is not far from 5(J 1 o0 of an inch for eleven in- 

 tervals. For a large number of intervals the deviation will 

 be somewhat larger, but it will not be proportioned to the 

 number of intervals. 



Microscopic Tracings of Lissajous Curves. 



Mr. R. G. West lias been successful in tracing these curves 

 on glass, in lines 55,000 to the inch, and in some respects they 

 are considered better fitted for microscopical tests than paral- 

 lel rulings. Aside from their great beauty, and the necessi- 

 ty for skilful illumination to display them well, the intersec- 

 tion of some lines and the gradual approximation of others, 

 arising from the variation in the figures, where every degree 

 of the sharpness of a curve is obtainable, from a line return- 

 ing almost upon itself at an exceedingly acute angle, to 

 curves so flat as to present in parts virtually the appearance 

 of parallel straight lines all this, combined with a knowledge 

 up to a certain point of the nature of all lines cut in glass, 

 makes these rulings more instructive, perhaps, than the mark- 

 ings on diatom valves, about which their is so much ques- 

 tion. A curious feature of some of these figures is, that 

 though all the lines would seem to be in the same plane, it 

 sometimes happens that an alteration of focus is requisite to 

 bring out the transverse lines. The same fact has been no- 

 ticed in observing the transverse markings of the Diatoma- 

 cew {Journal of the Quekett Microscopical Club, July, 1878). 



