MEDIUM-POWER PHOTO-MICHOGRAPHY 121 



with the coarse rack, being subsequently completed by the aid of the fine adjustment, 

 which is turned (through the medium of the silk cord) by the brass handle shown in 

 the diagram, running beside the camera. We then arrange the shutter so that it will 

 drop a sufficient depth — such distance being regulated by the stop-screw— to cover 

 the entire substage condenser. The light is then temporarily turned down by means 

 of the " cut-ofP" handle described when explaining the jet, whilst two plates, if not 

 sold as such, are being " backed " previous to placing them in the dark slide, which, it 

 may be here stated, can be done at once, even if they are not quite dry. The light is 

 then again turned up, and, having given one final glance at the ground glass to make 

 us feel assured the light has remained equally distributed over the field, and that the 

 focus is perfect, the exposure is made. It is best to note the exact time by a watch 

 for this object, under the circumstances narrated about two or three seconds. Note 

 in lowering or raising the exposure -shutter not to let it strike the microscope. 



The second plate is then exposed in a similar manner, giving it double the time of 

 the first, as by that means the operator is able to judge more accurately the correct 

 exposure. 



It is possible when these pictures are developed (about how to do this we 

 shall shortly speak), that although they are sharp there is still a lack of contrast ; 

 each negative looks flat, whilst the object does not " stand out " in the way it should. 

 This is probably due to the fact that too much light has been admitted to the 

 objective— it has been what is called " flooded." To obviate this we had better look 

 at the object again and see if by shutting down the iris diaphragm a trifle we do not 

 get a better contrasted eflect. It must not be so closed as to produce defractive 

 phenomena — such as bright lines around the hairs. Fig. 3, Plate III. — for if so we 

 obtain a photograph of optical phenomena, and not one of the proboscis of a blow-fly 

 If, however, we close our iris as much as we dare and yet are not satisfied with the 

 picture on the ground glass, which experience will soon teach us to see at a glance, we 

 must gently close the Davis' diaphragm situated above the objective, just at the end 

 of the microscopical tube. This must be used with much caution, for a very small 

 amount of closing will produce a great effect upon the picture, and it will be entirely 

 spoilt if closed too much. If the objective be a good one, and the eye-piece equally 

 so, and all extraneous light excluded from the camera at its juncture with the micro- 

 scope, our picture should be now one full of brilliancy and sparkle. Fig. 8, Plate III., 

 and Fig. 4, Plate III. 



Another good object somewhat similar is the leg of a bee ; it is shown in Fig. 6, 

 Plate III. 



Q 



