16 ON THE DIFFERENT METHODS OF MEASURING 



Having completed by this method the drawing of the object, or 

 simply indicated the extremities, the application of a metrical 

 measure "will immediately give the dimensions, increased precisely 

 in proportion to the magnifying power used in the microscope ; 

 from which it results that the real size of the object is equal to the 

 apparent size divided by the magnifying power, in diameters. It is, 

 however, essential to remember, that in order to obtain with the 

 Camera Lucida, a drawing corresponding in dimensions to the in- 

 crease obtained in the field of vision, the distance from the reflecting 

 point to the plane in which the drawing is being made must be 

 precisely equal to the distance from the same point of the object 

 under observation ; while the drawing and the measure which is 

 deduced from it will be greater or smaller than the dimensions 

 presented by the object in the field of the instrument in proportion 

 as the height of the Camera Lucida from the plane of the drawing 

 may be greater or less than is correct. 



Such is the most prompt and most practical method of determin- 

 ing the dimensions of objects which are observed under the mi- 

 croscope. Still, such a determination is a thing of very little im- 

 portance, if not even a mere matter of curiosity. The same cannot 

 be said, on the other hand, of the number of stride, or rows of dots, 

 or of cells, which may occupy a given space on the surface of the 

 observed object. Although up to the present time those who are 

 engaged in observing the Diatomaceze are not agreed in recognising 

 the importance of such a datum in regard to the value it may have 

 as a diagnostic character, yet it is generally acknowledged that, at 

 any rate, within certain limits of variableness, the number of the 

 stride which cover the valve of a Diatomaceae, is one of the means 

 of identification of the species to which it belongs. It is, however, 

 useless to trust to the means of the Camera Lucida for determining 

 the thickness of the striee, so close together and so fine are they ; 

 for this purpose it is necessary to have recourse to means of much 

 more exquisite delicacy. 



A process answering better for this purpose is found in the use 

 of the eyepiece micrometer, which is nothing but a measure adjusted 

 to the eyepiece, from which measure is determined the value, in re- 

 lation to the magnifying power used by means of a millimetre cut 

 on glass, and divided into hundredths and placed under the objec- 

 tive. The commonest form is that of a thin plate of glass, on 

 which is engraved a series of equal divisions, and in order to 

 facilitate the reading every fifth and tenth mark is longer than the 



