29 



in the short time at our disposal, but as several members have 

 signified a desire to this effect, perhaps a few short remarks may not 

 be out of place ; at the same time, I do not propose to fully enter into 

 the details of this important application of the microscope to the 

 reproduction of the microscopical image. Drawings and diagrams are 

 excellent in their way and in many cases indispensable, but for 

 accuracy of delineation they are not to be compared with the results 

 obtainable by photography. Elimination of the personal element 

 is also a factor that should not be overlooked. 



In the apparatus employed the microscope is brought to the 

 horizontal position. The object may be illuminated by a paraffin 

 lamp, the rays from which pass through a bull's eye condenser to the 

 substage condenser and thence through the object. (This method 

 applies to transparent objects.) The photography of opaque objects is 

 effected under the conditions I have already described under the head- 

 ing of "Dark Ground Illumination." To secure an evenly illuminated 

 disc the method of procedure is similar to that employed in the optical 

 lantern. The source of light, be it a paraffin lamp, incandescent gas, 

 electric lamp, or limelight, must be exactly central with the condensers 

 and the axis of the microscope, a little adjustment of the bull's eye con- 

 denser, either nearer or farther away from the source of light, being all 

 that is necessary for obtaining an evenly illuminated disc on the screen 

 or sensitive plate. The image of the object is now focussed on the 

 ground-glass screen of the camera by means of a focussing rod con- 

 nected with the fine adjustment. An eyepiece is unnecessary for 

 low-power work. It will be found in practice that the ground glass 

 is too coarse for accurate focussing, so we substitute for it a piece of 

 plain glass and focus the aerial image with an ordinary photographic 

 focussing lens, to obtain the necessary definition. The beginner 

 will be well advised to attempt at first only low-power work, for in 

 using high powers difficulties of a technical character crop up which 

 would tend to discourage him. As a general rule when photograph- 

 ing insect preparations a coloured screen is not required, although 

 the latter is very necessary for bacteriological or other stained 

 objects. If the object is of a dark, chitinous character, the difficulty 

 may generally be surmounted by giving increased exposure. Isochro- 

 matic plates are a necessity and they should be " backed." Develop- 

 ment is carried out to full density for correct exposures, hence a 

 fairly strong pyro developer with plenty of sulphite will be found to 

 give the best results. It is difficult to give precise particulars as to 

 the length of exposure required, it depending on the character of the 

 subject, source of illumination, the kind of lens used, and the 

 magnification. 



It is important to keep an indexed record of the subjects photo- 

 graphed, giving such particulars as — name of specimen, generic and 

 specific, whether male or female, where obtained, how prepared, 

 the objective used, amount of magnification in diameters, exposure 

 and date, and any other fact that requires notice. Such a record 

 will prove invaluable for reference in the future. 



