144 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



with a ''Low" pneumatic shutter, this model being chosen on account 

 of the small amuuut of space it occupies, allowing it to be introduced 

 between the camera and the microscope. This was accomplished by 

 clamping the shutter to the collar on the front-board of the camera, 

 another similar collar being screwed to the front of the shutter for the 

 light tight connection with the microscope. With the microscope in the 

 horizontal position the light was taken directly from the mirror of 

 the heliostat and diffused by means of a disk of blue ground-glass 

 placed in the substage immediately behind the iris diaphragm, and then 

 passes through a simple condensing lens to the object on the stage of 

 the microscope. The immediate source of the light, the ground-glass, 

 is thus brought near to the object to be photographed, giving a brilliant 

 illumination and permitting the use of a small diaphragm. 



The most difficult task was to confine the animals to be photographed, 

 more particularly with the microscope in the horizontal position. The 

 device which proved most serviceable for flat or discoidal objects was a 

 parallel compressor of the model of Hermann Fol. Rings were cut 

 from pure rubber tissue of diflferent thicknesses, the ring to be em- 

 ployed for any particular object being a little thicker than the object 

 itself. The rubber ring was then placed on the lower plate of the com- 

 pressor and pressed into contact with it by means of the finger. The 

 object is then brought into the centre of the ring, and water added with 

 a pipette until the inside of the ring is completely filled, and the upper 

 part of the compresser carefully screwed down until it comes in contact 

 with the rubber, the superfluous water being at the same time squeezed 

 out. If this be done with care, the inside of the ring will be com- 

 pletely filled with water and contain no air bubbles. There should be 

 just enough pressure to allow the upper glass of the compressor to come 

 in contact with the object. This can be determined by holding the com- 

 pressor vertically, and screwing down the upper plate until the object 

 ceases to sink. The compi-essor can now be clamped to the stage of the 

 microscope in any position. By employing rubber rings of sufficient thick- 

 ness, aquaria can be contrived in this way one eighth of an inch in depth. 

 In photographing the rounder and plumper forms, any pressure upon 

 the animal would produce a sensible change in shape. Such forms, 

 therefore, were placed in small deep watch glasses and confined by 

 glass rings, the microscope being placed in a vertical position, the camera 

 however remaining horizontal. The connection between the microscope 

 and camera was efi'ected by means of the prism end of an Oberhauser's 

 camera lucida, to which the light tight collar had been adjusted by 



