ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 239 



or an ocular requiring a minimum distance of 24 cm. is used. Means 

 are contrived for rapidly fixing the films in the proper positions. A 

 micrometric objective photographed once for all defines each magnifica- 

 tion. A prism is so placed that it is possible to follow the position of the 

 object on the film for the purpose of keeping it in the field. This con- 

 venience is, naturally, very useful in the case of a rapidly moving body. 

 The observations were in three stages : 1. From impregnation to com- 

 mencement of the cavity of segmentation, 8 hrs. 2. From the cavity 

 of segmentation to the first movements of the blastule, 3 hrs. 40 mins. 

 3. From the first movements of the blastule to its liberation, 1 hr. 20 mins. 

 The lengths of film used were respectively 54, 67, and 26 metres. The 

 images were 53 per metre. The plates accompanying the paper give 

 selections of the results. 



Axial Illumination in Photomicrography.* — D. J. Reid said, in 

 order to obtain the best possible results in photomicrography, critical 

 illumination should be employed ; that is to say, that the image of the 

 source of light, and the plane of the object to be photographed, should 

 both be in the focus of the objective at the same time. There are, 

 however, two requirements which are desirable, more especially in 

 photomicrography, in connexion with this so-called "critical illumina- 

 tion," the first being that tlie field of view should be uniformly and 

 fully illuminated ; and the second that the whole of the back combina- 

 tion of the objective should be filled with light at full aperture — i.e. 

 that the full aperture of the lens should be capable of being used if 

 necessary, which is only possible with certain objects. For the purposes 

 of photomicrography with critical illumination we must use a collecting 

 lens, as here we require the whole field uniformly and fully illuminated. 

 Granting this, there are a few fundamental principles relating to the 

 illumination of the field, and concerning the filling of the aperture of 

 the objective with light, that we ought here to refer to : — 



1. As to the illumination of the field : — 



A. The extent of the illumination, when using the same ocular and 

 objective, depends : — (n) On the diameter of the collector, (b) On its 

 distance from the Microscope. (The farther it is away the smaller is 

 the area of the field illuminated.) (c) On the power of the substage 

 condenser. The higher its power, the smaller will be the area of the 

 field it will illuminate. 



B. Next, the intensity of the light, when at a fixed distance, depends : — 

 (a) On the nature of the source of light. (Whether kerosene lamp, acety- 

 lene or electric light, etc.) Personally, I use the Nernst lamp, with single 

 filament, which is intense enough for most purposes, {b) On the Avay in 

 which a flat flame is used, edge or flat, (c) On the angular aperture of 

 the collecting-lens, and on its corrections, (d) On the angular aperture 

 of the substage condenser. 



2. In the same way as we considered the fundamental principles 

 relating to the illumination of the field, we ought now to enumerate and 

 shortly consider those that govern the filhng of the aperture of the 

 objective with light. 



* Abstract of lecture delivered to the Photomicrographic Society, on Jan. 10, 

 1912, and reported at nearly full length in the " English Mechanic " of Jan. 19. 



