20 ON MEASURING MICROSCOPICAL OBJECTS. 



another. The images which I obtain directly, and which are usually 

 called by photographers, negatives, from which I print off so 

 many identical proofs, are obtained on plates of glass. These 

 present with the greatest fineness and fidelity, not only the form of 

 the Diatomaceae, but even the finest details, which can only be 

 perceived with doubt and great trouble when observing them di- 

 rectly under the microscope. Would that we could obtain equally 

 fine results in the positive which is printed on paper, and which, 

 by the imperfection of the surface presenting small asperities of ele- 

 vated and depressed points, does not perfectly and equally ad- 

 here to the glass of the negative, and is far from presenting the 

 same degree of fineness ! Having thus at my disposition the most 

 faithful and authentic representation of the Diatomaceae on a plate 

 of glass, I direct my attention to this, and on it I follow the enu- 

 meration of the striae. To facilitate the enumeration I count the 

 lines which correspond to a hundredth of a milimetre, multiplied 

 five hundred and thirty-five times, and I see how many lines are 

 included in a space of the negative equal to 5. 35 m m . This 

 measure, cut on a thin metallic plate and placed on the negative or 

 matrix, gives the greatest facility for obtaining the number which, 

 multiplied by a hundred, will give as the final result, the precise 

 number of stria?, or rows of dots, which cover the valves of the 

 Diatomacese. 



Those, however, who are not able to have recourse to Photo- 

 Micrography, which also offers the incalculable advantage of giving 

 as it were, an authentic reproduction of the objects under study, 

 in order to be as near as possible to the truth, ought to have re- 

 course to repeated measurements according to different methods, 

 assuming for the final result, the mean number obtained by the 

 repeated operations. 



Translated from the Italian, Nov. 16th, 1869. 



