120 



PHOTO-M I CROGR APH Y 



must be made so by using the adjusting screws. Re-open now the iris, and re-obtain 

 critical light, and adjust the lime once again till its brightest part is seen in the 

 centre of the field. The image of the lime becomes now very objectionable, and far 

 too bright for the eye to stand, so it is best at this juncture to place a green glass 

 between it and the condenser. Lowering a condenser practically reduces its working 

 aperture, but seeing the aperture of the condenser in use is higher than that of the inch 

 objective, we may lower it until the field is equally illuminated without fear of losing 

 definition. Still, if we desire, for some special effect or reason, to use the absolute critical 

 image, we must broaden out the narrow image by the use of the auxiliary lens,, placing 

 it in position as shown in the diagrana. Fig. 24B page 54 (about two inches from the 

 lime), moving the rack adjustment up and down and from side to side until the light is 

 evenly distributed over the field. The substage iris may have to be reduced in aperture 

 when using the inch objective or the object may suffer from what is called "flooding 

 of light." It is not often that critical light is required to be used with so low a power, 

 for the simple reason that an inch objective is mostly employed to give a general view 

 of an object, the relation of the many parts to the whole, rather than the details of 

 any particular portion. The principal occasion with an inch objective for requiring 

 critical light is when photographing the general appearance of a minute object like a 

 diatom, in which case the camera may have to be specially increased in length, even 

 perhaps, to as much as six feet. Magnification, then, is the only object, for the N. A. 

 of the inch is far too small to give much detail. Still, if it be desired to use the lens 

 to the best advantage, critical light must be employed, and as the substage condenser 

 will not spread out the light all over the entire field, the auxiliary condenser must be 

 brought into use to do so effectually. 



Having then centred the condenser and the lime, and broadened out the light if 

 necessary, the green glass may be removed and the camera placed in its slides, as 

 shown in the diagram. Figs. 24 a and B, page 54, taking care that the piece of brass 

 tubing on the camera, which slips loosely into that portion attached to the microscope, 

 does not touch it at any j^oint of its circumference. The camera is then drawn out to 

 10 inches or more as desired. It is firmly fixed to the table by the wooden screws. 

 Then, if we are not using the auxiliary condenser, we ask the assistant to turn the 

 lime just a touch from side to side or up or down till we feel assured there is equal 

 illumination ; and, if the auxiliary condenser is added, the same process must be 

 repeated again by shifting it (by means of its two racks) until the light is equally 

 illuminated over the whole field as before described. 



Focussing is the next operation. When employing the inch, it is best done at first 



