14 



THE AMERICAN MONTHLY 



[January, 



convenience of printing are secured. 

 The end may be readily accomplished 

 by matting the plate as a negative is 

 matted in printing. 



Suppose it be desired to photo- 

 graph four different species of acari 

 on one plate, the image of each 

 v^hen magnified to the desired extent 

 only covering about one-fourth the 

 exposed area of the plate. First a 

 mat is prepared of card-board or thick 

 non-actinic paper w^hich 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 dis- 

 tance 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 re- 

 versed both sidewise and endwise, 

 and the lugs will fit the notches ; if 

 so, it is ready for use. The object 

 being focussed upon the focussing 

 glass or card, the camera is i-aised 

 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 focussing glass is in- 

 serted in the position the plate 

 will occupy and final adjustment ^ 

 and focussing made. The plate '^~ 

 is then marked on one corner 

 on the film side with a lead 

 pencil, placed in the holder with- 

 out disturbing the mat, and the 

 exposure made. When the plate 

 is replaced for a second expo- 

 sure either the mat is reversed or the 

 plate turned end for end ; but it is 

 best to always place the plate in the 

 holder in the same position and 

 change the mat to expose successive 

 quarters, but this requires the camera 

 to be moved for each exposure. 



With similar objects, and some 

 judgment in making the exposures, 

 negatives may be made with almost 



exactly the same density in each 

 quarter, and by cutting out slightly 

 less than one-quarter of the mat the. 

 four images will be separated by 

 black lines in the print ; by cutting 

 out a trifle more than the exact 

 quarter they will be separated by 

 white lines instead of black. 



The Iris Illuminator.* 



BY R. H. WARD, M. D., F. R. M. S. 



One who has employed the various 

 forms of graduating diaphragm can 

 scarcely fail to appreciate the luxui"y 

 of being thus able to regulate with 

 precision the amount of light em- 

 ployed, and the breadth of the illumin- 

 ating pencil, without abrupt change, 

 and while the object is under uninter- 

 rupted observation. Heretofore, how- 

 ever, this expedient seems to have 

 been applied only to axial illumina- 

 tion. For the sake of attaining simi- 

 lar advantages with oblique illumina- 

 tion, the writer has devised and used 

 a combination believed to be new, 

 which, for want of a more correct 

 name, he calls the " Iris Illuminator." 

 It consists, as shown in Fig. 7, of 

 any desired lens system, either dry or 



Fig. 7. — Iris Illuminator. 



immersion, under and close to which 

 is mounted an iris diaphragm, with 

 a decentering adjustment ; the dia- 

 phragm being set in a sliding plate 

 moved by a screw or lever, so that it 

 can be moved into any position from 

 the centre to the periphery of the sys- 

 tem, without altering the position of 



* Read before the American Society of Microscop- 

 ists, 1884. 



