60 Proceedings of the Royal Irish Academy. 



roughly focussed, it is then by means of the mechanical stage removed 

 from the field, and the diaphragm aperture which is intended to be 

 used in the particular case having been placed in position, the achro- 

 matic condenser and light are manipulated until the field is evenly 

 illuminated ; the diaphragm plate is then revolved until the full open- 

 ing is reached ; the object is then brought back into position, and the 

 best possible image obtained by means of the fine adjustment of the 

 microscope : the diaphragm plate is then again returned to its former 

 position ; the image, of course, gains much in sharpness, and although 

 quite sufficiently bright to produce an impression on a rapid plate, is 

 not at all in as satisfactory a condition for accurate focussing as when 

 presenting a brighter appearance. 



When all the adjustments have been made, the sleeve suspended 

 from the frame is placed in position, one end of it being attached to 

 the sliding front of the camera, and the other end to a pasteboard 

 cylinder, which fits on to the back of a narrow box, containing a 

 sliding shutter by which the exposure is made ; to the front of this 

 box the body of the microscope is attached by a small black velvet 

 sleeve which completes the camera : the large sleeve is made of Mack- 

 intosh cloth with three hoops fastened inside to prevent its collapsing. 

 The camera body and microscope are placed on a simple frame- 

 work, which is grooved on top to allow the camera body to slide to 

 any desired distance from the microscope. In order to obtain a con- 

 stant amount of magnification with each objective, I mark on the 

 framework the position of the camera body corresponding to so many 

 diameters for each objective, this being ascertained by placing a micro- 

 meter on the microscope stage, and measuring the image on the screen 

 with compasses. 



All the photographs done by me lately have been taken on plates 

 prepared by Mr. Fredman, chemist to Messrs. Bewley & Draper, and 

 they have given me great satisfaction in every way. 



As regards the exposures necessary, of course they will vary with 

 plates of different makers : the plates which I have used are not so 

 rapid as some made for instantaneous work, but as they are quite rapid 

 enough and present several advantages, at least in my hands, I have 

 preferred them to others of greater rapidity — in fact, the excessive sen- 

 sitiveness of some plates is a positive disadvantage for this work, one 

 of the defects complained of by those who have used them being the 

 so-called "halation," which I have never had to complain of with 

 Fredman plates. I give the shortest possible exposure necessary to 

 secure a dense picture, and the pictures shown to-night were printed 

 from negatives which received an exposure of 60 sees, for the 1-inch 

 objective, and 90"-120" for I" obj. when using the lime-light. 



I have not yet used an objective of higher power than \" : this 

 gives a well-defined and satisfactory image of 200 diams. at a distance 

 of about 30 inches from the screen (no eye-piece being used) and the 

 pin-hole cap on the achromatic condenser. Similarly the 1" obj. gives 

 an image of 50 diams., with a similar distance between it and the 



