ON MEASURING MICROSCOPICAL OB ECTS. 15 



much as I have been able to collect from the best treatises which 

 have been written on the subject : I shall, however, speak from 

 experiments made by myself, to which I have united a few practical 

 particulars which from time to time I have ascertained to be the 

 most useful. I hope however, it will not be altogether useless to 

 make known the means I make use of, thus, as it were, clearing the 

 way for those who wish to undertake similar researches, or who 

 wish to become expert in the use of the microscope. 



From the first moment when I undertook the study of the Dia- 

 tomaceae, I was induced to occupy myself with their measurement 

 as one of the data which might serve for the determination of the 

 species, and for the identification of the subjects which I had under 

 my eyes, with the species described in Smith's Synopsis of the 

 British Diatomacese, and in the works of Kiitzing, Rabenhorst and 

 others. The easiest system for taking such measurements is that 

 founded on the use of the Camera Lucida, an invention which is due 

 to the celebrated English savant, Wollaston, in 1807, which has 

 been successively modified and perfected by the illustrious German 

 anatomist, Soemering, the French opticians, Chevallier and Nachet, 

 and the Italian Professor, John Baptist Amici, whose name is con- 

 nected with nearly all the improvements in the microscope. 



The Camera Lucida consists essentially of a reflecting surface, 

 which forms an angle of forty -five degrees with the axis of the 

 microscope, which surface must be such as to permit the simul- 

 taneous view of the object disposed in the field of the instrument, 

 and of the plane on which the same image is reflected. It is then 

 extremely easy for anyone whose hand is trained to the use of the 

 pencil, to draw, by these means, the object under observation, fol- 

 lowing all the contours, and making, as it were, a tracing. Some- 

 times, however, he finds some difficulty in seeing with ease, at the 

 same time, the reflected image, the contour already drawn, and the 

 point of the pencil which is completing the drawing. This incon- 

 venience arises from the want of a just relation between the illumi- 

 nation of the field of the microscope and that of the plane on 

 which he is drawing. Thus, if the former is very much illuminated 

 and the paper in shade, it will be difficult to have a simultaneous 

 view of the object and of the drawing he is making. Such an in- 

 convenience must be carefully obviated by diminishing the concen- 

 tration of the light in the field, or by selecting a position in which 

 the plane on which the drawing is being made will be better 

 illuminated. 



