MICROSCOPICAL OBJECTS. 19 



tion with which they are adorned, I usually approach the two 

 points of Hartnack's micrometer very close, so as to be able to 

 include in the interval only one or two stria3 which I judge I can 

 easily keep within view. Afterwards I substitute for the micro- 

 scopic preparation, in the plane of the object-holder, the objective 

 micrometer, with the micrometer cut on the glass, with the Camera 

 Lucida ; I draw the interval between the two points, and that of a 

 hundredth of a millimetre magnified in the microscope, and by ob- 

 serving the number of times that the latter is greater than the for- 

 mer, and multiplying it by the number of strife observed between 

 the two points, I obtain the number of strife contained in one hun- 

 dredth of a millimetre, and consequently the number contained in 

 a millimetre. 



All these systems certainly give an approximative idea of the 

 measures, but we cannot expect from them an exact and precise 

 determination, and this so much the more in proportion as the 

 basis of the calculation maybe smaller; — seeing that any error 

 there may be in the first place, though only of a fraction of a stria 

 by being multiplied as many times as the smallest space is com- 

 prised in the measure of the millimetre magnified in the micros- 

 cope, may amount to a sufficiently notable difference from the 

 truth. And this I think may be the origin of the differences in the 

 number of the striae which are observed in the Diatomaceae of the 

 most distinguished micrographers, who purposely and specially, or 

 only accidentally, have occupied themselves with the Diatomacete, 

 in order to the determination of the species to which they 

 belong. Having, however, proposed to occupy myself principally 

 with the study of this interesting class of organisms, I was led to 

 think of some method which, by facilitating the operation, would 

 lead me to a more exact estimation, rendering it not only possible, 

 but relatively easy, to count the striae, and in consequence deter- 

 mine their sizes and that of their intervals. 



Such a method I find in the habitual use of Photo-Micrography, 

 by which means I reproduce the different forms which present 

 themselves in my researches. Having proposed to myself to edit a 

 most complete photographic monograph of the whole order of the 

 Diatomaceae, in which I have up to the present time included 

 nearly a thousand types, I adopted the magnifying power of 535 

 diameters for the reproduction of these, so that I could keep an 

 approximative account of the relation of size between one type and 



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