37 



and Wales 1 -8th with amplifier, he satisfactorily resolved the 

 29th and 30th bands of Robert's test plates, the lines of 

 which were 1-83,917 and 1-86,334 to the inch. 



Dr. Woodward lately received a Nobert's test plate of 19 

 bands, the photographs procured from wliich were lately 

 brought under the notice of the Royal Microscopic Society by 

 the hon. secretary, Mr. Jabez Hogg, the covering glass on this 

 test-plate was too thick for a l-50th objective, but with all 

 the others he was able to resolve the 17th band, 101,000 to 

 the inch, the 18tli and 19th he was unable to resolve. Both 

 Dr. Curtis and Dr. Woodward have photographed the 16th, 

 17th, 18th, and 19th bands with a Powell and Lealands' 1-25. 

 In these photographs the lines of the 16th and 17th bands 

 can be counted with some dif&culty, but the lines of the 18th 

 and 19th cannot. 



Dr. Barnard found the counting of the lines to be attended 

 with a great difficulty, in addition to which there is another 

 trouble, the whole width of a band is not in perfect focus at 

 the same time, and the slightest change in focal adjustment 

 renders it extremely difficult to fix, even with the cobweb mi- 

 crometer, the last line counted. 



One way of verifying the accuracy of the divisions of 

 Nobert's test lines, is by counting them in a measured space. 

 If 46 equidistant lines are ruled in the space of l-2,000th of 

 an inch, the inter-spaces must be at the rate of 90,000 to the 

 inch. An error of one line more or less in counting the whole 

 on such a band, would decide the rate of interspaces to be 

 either 92,000, or 88,000, instead of 90,000 to the inch. 



The foot note to Mr. Stodder's paper in the " American 

 Naturalist," deserves notice. Speaking of Dr. Woodward's 

 photographs, the author says, " The first fifteen bands are 

 sharply and clearly resolved into the true lines ; the fifteenth 

 band, however, (which is ruled to the 90,000 of an English 

 inch), requires a hand-glass magnifying four or five diameters, 

 to show its lines distinctly." Dr. Woodward gives the 

 following directions for counting the lines in the highest 

 bands that can be resolved. " If a cobweb micrometer 

 is used, the micrometer eye-piece should be firmly clamped 

 in a stand screwed to the table, so that the eye-piece 

 is close to the end of the microscope-tube, but not to 

 touch it, a i^iece of black velvet being used to complete the 

 connection. The motion of the micrometer-screw now com- 

 municates no tremor to the microscope, and all difficulty in 

 counting the lines seen (whether real or spurious) disappears." 

 Still better than this is the following method : — " The micro- 

 scope being set up in a dark room, as though to take a photo- 

 graph, and the eye-piece being removed, the image of the 



