416 ANIMAL NATURE OF DIATOME^l. 



prominence, are certainly unequal. If we compare the 

 three figures of Navicula nodosa (PL iii, fig. 57 ; 1, 2, 3,) 

 we shall see in one of these (2) a median enlargement 

 that is wanting in the others. Certainly we find minor 

 differences when comparing species with species. If we 

 look to dimensions, it is only in some species that 

 Kutzing observes and figures the two extremes in size, 

 as, for example, in N. amphisbaena, (PL iii, fig. 42.) For 

 the rest he contents himself with noting the largest size. 

 But Ehrenberg observes also the smallest extreme that he 

 meets with, and delineating exactly the intermediate 

 forms, puts in evidence the specific characters that re- 

 main independently of age and degrees of development. 

 Treating of the dimensions of these microscopic beings, I 

 cannot avoid making a few observations. Mohl has 

 discussed the methods of micrometry in a profound 

 manner, showing the comparative degree of accuracy to 

 be attained by them. From that inquiry it appears that 

 the camera lucida is the most exact and safest of all ; I 

 have found it more convenient than any other. Taking 

 a glass micrometer (by Plossl) in which a millimetre is 

 divided into 100 parts, I copy the image of it by the 

 camera lucida, repeating the operation many times to 

 ensure the exactness of my copy. Though executed with 

 an excellent machine, the diamond-marks are never per- 

 fectly equidistant, .and are always too broad to exclude 

 slight inaccuracies. On this account many trials are 

 required to obtain a sufficient approximation. From this 

 copy I can ascertain with precision the amplification 

 obtained, which is always relative to the vision of the 

 individual. Whenever I wish to determine the size of 

 an object, I copy the image with the same combination 

 of eye-glass and object-glass, with the same camera 

 lucida, at the same distance ; and measuring that upon a 

 graduated scale, I obtain the dimensions sought for by 

 an easy reduction. To abridge and facilitate the inquiry, 

 I construct a decimal scale on a copy of the micrometer ; 

 upon this I can draw the divisions, if the magnifying 



