224 Transactions of the Royal Microscopical Society. 



tlie same objective ; tliey were made as nearly as was convenient 

 under the same circumstances ; corresponding parts of the different 

 frustules of the same species were selected for measurement ; and 

 the part selected was noted, so that, if the matter were worth the 

 trouble, reference could be again made to the identical frustule and 

 the identical part of it which was the subject of the present measure- 

 ment. The objective used was an immersion ^^th made by Tolles ; 

 when used under the conditions of these measurements, its focal 

 length was found to be sixty-two thousandths of an inch. The 

 micrometer used was a cobweb micrometer by Troughton and Sims, 

 of London ; the value of a revolution under the conditions obtaining 

 during these measurements was determined by the mean of twenty- 

 seven comparisons with a millimeter divided into a hundred parts 

 by Hartnack, and by a larger number of comparisons with a Paris 

 line divided into a hundred parts whose author is not known. Care 

 was taken to make the comparisons include the whole line and the 

 whole millimeter in such a way as to eliminate the effect of 

 inequalities in division. 



For the first thirteen diatoms of the Probe-Platte, lamplight 

 was employed except for Platte No. 258 ; for Platten Nos. 481, 

 535, 572, 586, and for the unnumbered Platte marked A, lamp- 

 light was also employed for diatom No. 14 ; for all others, sunlight 

 was reflected by a heliostat through cobalt glass upon the concave 

 mirror of the microscope. In measuring Platte No. 258, the 

 micrometer was almost always in the tube of the microscope ; 

 with Platten A and 572, it was always mounted in a separate 

 holder so that the micrometer could be manipulated without 

 communicating tremors to the image of the object measured ; 

 in the case of the rest, the micrometer was mounted separately 

 whenever sunlight was used for the illumination. When the 

 micrometer was in the tube of the microscope, its wires were made 

 to coincide with two striae and the number intervening was counted 

 two or three times ; when it was in a separate holder, the wires 

 were commonly made to coincide with the same stria, and one of 

 them was then moved over a certain number of striae which were 

 thus counted. 



Of the somewhat more than five hundred measurements thus 

 made, three have been suppressed because they did not agree with 

 other concordant results ; some measurements it seemed superfluous 

 to communicate, in which case the extremes have been given. 



In the case of the diatoms from No. 2 to No. 10 inclusive, mea- 

 surements were made on two specified parts of each frustule. It is 

 thought that these measurements are mostly correct to within one 

 or two units in the first decimal place. In the case of the re- 

 maining diatoms, two or more measurements were made as nearly 



