MICROSCOPICAL OBJECTS. 17 



others. Such divisions fixed in the focus of the eyepiece are seen 

 ajross the field of the microscope, in such a manner as to see 

 contemporaneously and with the greatest fineness and distinctness 

 the object which it is wished to measure, and the divisions. Let 

 us suppose that the object has transverse lines, of which we 

 wish to know the intervals, in order to deduce what number 

 would be required to occupy the space of a millimetre. To ob- 

 tain this I arrange the object under the eyepiece micrometer, 

 in such a manner that the striae may be parallel to the divi- 

 sions of the micrometer, and by causing one of the divisions 

 to be superimposed, and to coincide with one of the striae, I 

 determine how many of them there are in the space of one or 

 more of the divisions, taking notice that the observation will be 

 the nearer to the truth in proportion as it extends over a greater 

 number of divisions. Knowing the value of the ocular divisions 

 in relation to the magnifying power used, with a very simple equa- 

 tion we obtain the number of the striae in the object corresponding 

 to one millimetre. 



Thus, for example, I take for observation a Diatom in the 

 shape of a boat, in which the central nodule is seen to be dilated 

 transversely, and I recognize it as a Stauroneis Phoenicenteron 

 (Ehrbg). The valves of this Diatom are ornamented with very fine 

 moniliform striae, and rows of granules. I wish to know their 

 thickness or the number of them which correspond to a millimetre. 

 For that purpose : 1st, — I adapt to the microscope a micrometer 

 eyepiece : 2nd, — I cause the division to be superimposed 

 on the lines of the Stauroneis, exactly combining one of them 

 with one of the divisions : 3rd, — I count the striae comprised be- 

 tween five divisions of the micrometer, and find they are exactly 

 eight. I ought now to ascertain the value of a unit in the division 

 of the eyepiece, and I obtain it by substituting for the preparation 

 placed under the microscope an objective micrometer, that is to say, 

 a millimetre divided into a hundred parts, and cut on a thin glass ; 

 and recognizing that in the magnifying power used forty-nine 

 divisions of the eyepiece correspond to six hundredths of a milli- 

 metre ; I establish the proportion : 49 : 0. 66 m m : : 1 : x. Thence 

 a^% m = 0.001224 m m , which is the value of a unit in our case. 

 Now, since eight was the number of the striae which were counted 

 in five units of the ocular micrometer, we ought to say : if in 

 5 x 0.001224 mm there are eight striae, how many are there in one 



VOL. II. c 



