ZOOLOGY AND BOTANY, MICKOSOOPY, ETC. 831 



by hand on the slide on whicli it rests. On reaching an approximate 

 focus, the stage is secured by a friction-screw. The fine adjustment 

 is controlled by a milled head placed directly under the ground glass 

 focusing-plate, and acts by lever on the nose-piece carrying the 

 objective, and thus tight cords ruining the fine adjustment fixtures are 

 dispensed with. The substage has a fitting to receive any ordinary 

 illuminating apparatus, and by a simple device a condenser can be 

 accurately centered. 



Actinic and Visual Foci. — Statements have recently been made 

 that with modern under-corrected microscopic objectives there is no 

 difference between these foci, and that no allowance need therefore 

 be made in focusing. Dr. J. D. Cox, while finding this correct for the 

 generality of objects, remarks that it docs not apply when large 

 amplifications are in question. When powers of 1500 and 1600 are 

 used for photo-micrography there is a distinct difference between the 

 two foci, and it is therefore still necessary to give particular attention 

 to the focusing in order to obtain sharp photographs. 



Compound Negatives.* — In photo-micrography it frequently 

 occurs that the operator, instead of devoting a negative to each of 

 two or more similar objects for comparison, printing both upon the 

 same print, prefers to have the whole series upon one negative, taking 

 from this a single print. There is often room for two or more images 

 upon the same plate. If the centre of the plate is devoted to one, 

 obviously no more can be accommodated on it, but by placing one at 

 each end, or one on each corner of the plate, both economy of plates 

 and convenience of printing are secured. C. M. Vorce points out 

 that this end may be readily accomplished by " matting " the plate as 

 a negative is matted in printing. 



Suppose it be desired to photograph four different species of Acari 

 on one plate, the image of each when magnified to the desired extent 

 only covering about one-fourth the exposed area of the plate. First, 

 a mat is prepared of cardboard or thick non-actinic paper, which is 

 adjusted to exactly fill the opening of the plate-holder, lying in front 

 of and close against the plate when exposed, and having one quarter 

 very exactly cut out. A convenient way to fit this mat is to leave 

 projecting lugs on each side at exactly the same distance from the 

 ends, and cut notches in the plate-holder into which the lugs may 

 closely fit. If this work is carefully done, the mat may be reversed 

 both sidewise and endwise, and the lugs will fit the notches ; if so, it 

 is ready for use. The object being focused, the camera is raised one- 

 half the vertical dimension of the plate, and displaced to one side half 

 the horizontal dimension, when the image will be found to occupy 

 one quarter of the plate. The mat being placed in the plate-holder, 

 a focusing-glass is inserted in the position the plate will occupy, and 

 final adjustment and focusing made. The plate is then marked on 

 one corner on the film side with a lead pencil, placed in the holder 

 without disturbing the mat, and the exposure made. When the plate 

 is replaced for a second exposure, either the mat is reversed or the 



* Amer. Mon. Micr. Journ., vi. (1885) pp. 13-4. 



