PEOCEEDINGS OF THE SOCIETY. 123 



Apres deux ans de travail, 'ces photographies furent reproduces par 

 la gravnre dans le bel atlas de microscopie qu'ils publierent en 1841. 



C'est avec un microscope analogue a l'ancien microscope solaire que 

 Foucanlt commen9a ces belles photographies, mais fatigue des variations 

 constantes de la lumiere solaire, il imagina d'appliquer la lumiere 

 electrique, et crea alors, la premiere lampe a point lumineux fixe et le 

 microscope photo-electrique a condensateur, decrit dans la preface de 

 l'atlas, par son maitre et collaborateur le Dr. Donne. 



Ces photographies sont done contemporaines de Daguerre et de la 

 premiere application de la lumiere electrique aux etudes scientifiques. 



Ces plaques originales qui sont venues en ma possession, peuvent 

 soutenir la comparaison avec les reproductions microscopiques faites 

 actuellement ; elles sont presque toutes datees et signees de la main de 

 Leon Foucault. 



C'est une vraie satisfaction pour moi, M. le President, de pouvoir 

 enrichir la collection de la Societe Microscopique par la presence de ces 

 temoins des efforts faits par les anciens micrographes. 



Veuillez croire, M. le President, a mes sentiments les plus respectueux. 



Alfeed Nachet, F.R.M.S. 



Some excellent micro-photographs of Diatoms and Podura scales were 

 sent for exhibition by Mr. T. A. O'Donohoe, who states they were all 

 taken with a Swift Portable Stand, Zeiss Apochromatic Objective 2 mm. 

 N.A. 1 • 30, Oil Lamp, and a very cheap enlarging Camera. 



Mr. O'Donohoe also sent some remarks made by Mr. Nelson upon 

 ■these photographs, from which the following extracts have been made :— 



The Amphipleura pellucida is an excellent photograph of the so-called 

 striae ; the black lines are nice and thin. 



The SurireUa gemma. — The black dot picture is excellent. 



The Podura scale is excellent, especially in the upper part to the 

 left hand. The structure upon the membrane is shown. This proves 

 that the focus was right. 



PUurosigma angulatum. — The x 2000 photograph is a long way 

 the best. There is a part where the silex is admirably rendered with 

 round holes in it and slightly darker edges. I think this is as good a 

 representation of angulatum as any I have ever seen. You will notice 

 that this is really a black dot picture, for if the hole had been smaller 

 the dark fringe of the edge of the hole would have met and made a 

 black dot. 



The size of the fringe depends upon the working aperture, that is, 

 upon the size of the disk of light illuminated at the back of the objective. 



The white dot images at the top and bottom of the photograph are 

 ghosts, the focus being too long. The bit near the nodule is good, but 

 not so good as the part indicated above, because it exhibits a tendency 

 to ran into lines. 



