ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 367 



exactly in front of the lens, and in a central position. A small well- 

 made bos must be carefully constructed to carry the prisms and objec- 

 tive ; or better still, a short wooden cylinder. In either cape the interior 

 must be perfectly smooth and painted a dead black, and at one end must 

 be fastened a lens-flange for attaching tbe box or cylinder in such a 

 manner as to be immediately behind tbe objective and as near to its 

 back lens as possible. Tbe prisms should be fitted witb two adjusting 

 s :rews capable of being manipulated from the sides of the box or 

 cylinder, so that the angle at which the prisms are inclined to one another 

 may be altered if necessary. Fig. 81 sbows the apparatus in position: 

 S is the adjusting screw of prism ; the micro-objective ; and H the 

 stage for carrying specimen. Fig. 82 shows the apparatus in place. 

 A telescopic partition must be constructed to run through the length 

 of the camera in an exactly central position. To obtain the best results 

 a full exposure should be given, so as to obtain vigorous negatives full 

 of detail with good contrasts.* 



Panoramic View Camera. — In 1850 Andrew Pritchard, on p. 83 of 

 his Appendix to Optical Instruments, in Natural Philosophy, Library of 

 Useful Knowledge (1832), says, " In taking panoramic views M. 

 Lerabours has ingeniously constructed a camera, the lens of which re- 

 volves about a vertical axis ; the plate is curved, and a screen is inter- 

 posed, which revolves with the lens ; this screen has a slit opposite the 

 centre of the lens, so that only a small part of the image impinges upon 

 the plate at one time. By properly regulating the time and motion, to 

 suit the varied brilliancy of the different parts of the landscape, and also 

 by making the slit opposite the sky narrowest, a very j>erfect panoramic 

 view is obtained." 



It is needless, says E. M. Nelson, to point out to photographers that 

 this description, now fifty years old, is precisely applicable to one of the 

 latest forms of cameras, thereby proving that " there is nothing new 

 under the sun." 



If a similar adaptation were to be made to a microscopic camera, the 

 object would have to be mounted upon a curved slide, while the negative 

 plate remained a plane surface. This might be useful in taking low- 

 power instantaneous photomicrographs of long and narrow objects such 

 as living worms. 



(5) Microscopical Optics and Manipulation. 



The Black and White Dot Phenomenon.! — J. Rheinberg suggests 

 that the origin of the black and white dot phenomenon in diatoms arises 

 primarily from the fact that the perforations in a diatom form approxi- 

 mately vertical partitions between two media differing in refractive 

 index. A certain portion of the light which impinges from below on 

 the partition from the side of the denser medium, is reflected as at the 

 surface of an ordinary mirror. Such reflected light is that which reaches 

 the plane of partition beyond the critical angle. In consequence dark- 

 ness is brought about on the less dense side of the partition in the space 



* This method is essentially the same as that described by E. It. Turner, Illus- 

 t'ated Ann. Microscopy, 1930, p. 52. 



t Journ. Quvk. Mini-. Club, viii. (Nov. 1901; pp. 113-8 (6 figs.). 



2 b 2 



