ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 719 



On account of some residual spherical aberration, the condenser 

 will probably have to be racked up a little to secure the greatest 

 amount of light. 



In fig. 170 the expanded edge of the flame covers the Tnceratium. 

 When the whole aperture of the condenser is opened the size of that 

 disk will not be altered. Its intensity only will be increased. When 

 the central stop is placed at the back of the condeuser, only in that 

 part of the field represented by the disk of light will the objects be 

 illuminated on a dark ground. But some will say : Suppose the disk 

 does not cover the object ; what is then to be done ? Simply this : 

 bring the lamp nearer the mirror. 



The size of the disk of light depends on three things. 



1. The diameter of the bull's-eye. 



2. The length of the path of the rays from the bull's-eye to tho 

 substage condenser. 



3. The magnifying power of the condenser. 



If 1 and 3 are constant ; the only way of varying the size of the 

 dark field is by 2, as already stated. 



The intensity of the light in the disk depends also on three things. 



1. The initial intensity of tho illuminant. 



2. The angular aperture of the bull's-eye. 



3. The angular aperture of the substage condenser. 



Mr. Nelson has elsewhere insisted that the power and aperture of 

 the substage condenser should bear some proportion to the power and 

 aperture of the objective used, and does not enlarge upon this, but 

 merely alludes to it, as it does not legitimately come within the range 

 of his paper. Finally, he says he prefers to make the disk of light 

 no larger than necessary. If the whole field is required, he fills it ; 

 but if only a portion is wanted, then he reduces the size of the disk 

 accordingly. 



Mr. A. C. Malley* strongly disputes Mr. Nelson's recommendation 

 of a bull's-eye fixed to the lamp, and prefers one mounted on a sepa- 

 rate stand, which is easier reached and moved, and by which tremor 

 is avoided. The bull's-eye should be placed about 3^ in. from the 

 centre of the flame, the lamp being surrounded by a tin shade 

 having a small plane mirror behind the flame, and an orifice the size 

 of tho bull's-oyo in front. Tho bull's-eye is formed of two plano- 

 convex lenses (3^ in. focus) with their convex faces together. He 

 also uses a cell of ammonio-sulphate of copper in front of the mirror. 



Hawkins's Observatory Trough.f — Mr. R. Hawkins suggests an 

 improvement on Dr. Giles's Live-cell, | which he thinks will make 

 the apparatus so simple, that any one can make half-a-dozen in an 

 hour or less without extraneous aid. The arrangement consists in 

 the use of clips, to keep tlio glass cover on, made of a piece of brass 

 wire bent to fit the slide, and so as to have sufficient power to hold 

 the cover well in position. 



♦ Engl. Mecli., xl. (1884) p. 299. 

 t Sci.-GoHsip, \Siir),p. 133 (1 fig.). 

 X tSee this Journal, onte, p. IH.*}. 



