18 ON THE DIFFERENT METHODS OF MEASURING 



millimetre ? And the final result will be .oo6i 8 2mm= 13 °7- Whence 

 the stride of the Stauroneis Phcenicenteron are of such fineness 

 that 1307 of them are contained in a millimetre. 



Although such a method of taking small measures may be cor- 

 rect in theory, and in most cases is found the most practical, yet in 

 many circumstances of recognizing the finest details it becomes a 

 difficult and uncertain method. And this is specially the case when 

 one has to do with the most difficult Diatomacea3, the study of 

 which requires the most powerful objectives, and the most accurate 

 direction of the illumination. Anyone who has familiarised him- 

 self with the study of these can well bear me witness how the per- 

 ception of very fine strige requires a sustained tension of the visual 

 faculty, so that one frequently hesitates, and it is not always possi- 

 ble to recognize without hesitation and with certainty the number 

 of very minute stride which may be confined in an interval which, 

 relatively to the magnifying power used, and to the infinite small- 

 ness of the details, appears considerable. Such an inconvenience I 

 have been able partly to obviate by the use of variable eyepiece 

 micrometers. I have two of these, one with cobweb lines, con- 

 structed by Nachet, of Paris, the other with variable points by 

 Hartnack ; the two lines or threads of the first and the two points 

 of the second are separated from each other to a given distance, 

 which is determined in the first place by comparison with an objec- 

 tive micrometer. The Diatom being placed in the middle of the 

 field of vision under the variable micrometer, the number of striae 

 is determined, either at one glance or by very slowly advancing 

 one of the lines or one of the points, and fixing all the attention 

 on the point or line in motion, and on the crossing made succes- 

 sively by the stria3, from which it is possible to determine the 

 number. 



But still, this method presents a grave difficulty in the oscilla- 

 tion which is inevitably communicated to the instruments, so that 

 more striae seem to pass before or behind the moveable point, so 

 that here again I find myself in uncertainty and in the fear of 

 erring. Such difficulties cannot be overcome otherwise than by 

 rendering the eyepiece micrometer independent of the body of the 

 microscope, by fitting to it a supporting foot distinct from the 

 mounting in question. In the absence of such an arrangement, 

 when I am about to occupy myself with the Diatomaceae, which 

 are most difficult, on account of the extreme delicacy of the stria- 



