38 



band to be counted is received on a piece of plate-glass in tbe 

 plate-holder, and viewed with a focusing- glass, on the field- 

 lens of which a black point is marked ; as the focusing- glass 

 is moved on the plate from side to side, the black point is 

 moved from line to line. The lines may thus be counted with 

 as much ease and precision as though they were large enough 

 to be touched with the finger." 



These photographs support an opinion given by Mr.Wenham 

 many years ago, that the time would come when photography 

 would reveal minute detail much plainer than it can be seen 

 in the microscope. The reason of this is obvious. In photo- 

 graphy the object can be illuminated by highly condensed 

 sun-light, which will produce intense black shadows, quite 

 unendurable to the eye, and it was with such illumination 

 that these photographs were obtained with a Spencer l-12th, 

 and a Tollis l-5th, loth dry objectives. Dr. Woodward made 

 five counts of the 19th band, which gave him a mean of 

 110,820 to the inch. The number, according to Nobert, is 

 112,668. Dr. Barnard counts for the 15th band 91,545; 

 !N'obert 90,074. There is a similar alleged variation in the 

 striation of Diatomaceae, among individuals of the same 

 species, Fleurosigma fasciola, has been specially designated by 

 Dr. Wallich, as very inconstant in its markings, as well as 

 Fleurosigma strigosum, Navicula-rhoTriboideSj and Nitzschia 

 sigmoidea. 



With the l-8th and the l-12th by Eoss, and the l-20th by 

 Smith, in the possession of the Eoyal Microscopic Society, 

 and with a l-8th, a l-12th, a l-16th, and a l-25th, by Powell 

 and Lealand, all dry objectives, on a new 19-band plate, all 

 the bands beyond the 12th seemed imperfect, the lines were 

 not separated. But with a 1-lOth and a l-18th by Hartnarch, 

 of Paris, a 1-1 6th by Merz, of Munich, and a l-20th by 

 Nobert, all immersion ohjectives, straight and well-defined 

 lines were separated as far as the 15th band inclusive. Several 

 trials by sunlight proved that the lines were thus rendered 

 more visible, but the immersion objectives maintained their 

 superiority by all methods. 



Continental oj^ticians and men of science have been aware 

 of the merits of the immersion system for some years past ; 

 and, from its having received so little attention in England, 

 continental makers do not scruple to say that the English no 

 longer take the lead, either as opticians or microscopists. 



The advantage claimed for immersion objectives are :— 

 greater working distance between the object and objective, 

 increase of light, and superior definition and clearness in the 

 optical image, which image is obtained by much simpler illu- 

 mination than that required in using high-power, dry objectives. 



