360 THE president's address. 



only a J plate, and the height of the centre of the plate 

 above the board is the same as that of the optic axis of the 

 microscope, viz., Powell's No. 1, when in a horizontal position. 



The dark slide is one of Tylar's cheap metal double dark 

 slides, nsed as a single dark slide ; this is the smoothest work- 

 ing, and most suitable one for this purpose, that has yet been 

 designed. One has merely to compare the draught of this 

 metal slide, which costs 2s. 6d., with the most expensively made 

 wooden one, to be convinced of its superiority. 



Lightness is not only unnecessary, but is an absolute dis- 

 advantage in photomicrographic work, which in this respect 

 diifers from ordinary photographic work. 



The microscope is so well known that it needs no descrip- 

 tion ; it has been almost in constant use since it was pur- 

 chased in 1876, the movements are still perfectly steady, 

 and if anything smoother than at first. Quite recently 

 diagonal rack work has been fitted to it, but the necessity for 

 this alteration Avas not occasioned by any shake or loss of time 

 in the old rack work, but diagonal rack work allows a smoother 

 motion to be obtained with less pressure of the pinion in the 

 rack. We next come to the method of gearing the fine adjust- 

 ment to the fine adjustment rod. With this part of the apparatus 

 several failures were experienced. A differential motion was first 

 tried, next friction by rubber contact, thirdly the band, you 

 now see, tightened by a spring, fourthly a similar band 

 tightened by rubber, this was the first employed on this 

 apparatus, lastly the same band tightened by a screw hook ; 

 which has never failed me. The conclusion formed with 

 regard to this part of the ajDparatus is to avoid springs and 

 rubber, however used. The rod being of smaller diameter than 

 the head of the fine adjustment screw, the movement will be 

 somewhat slower ; of course, a perfect slow movement is a sine 

 qua non in high power photomicrographic work. The cord 

 and weight as described by Mr. Bousfield is also a steady 

 movement. The slow movement by the Campbell differential 

 screw in Mr. Baker's microscope is very steady and peculiarly 

 suitable for photomicrographic work ; not only is this my own 

 experience, but several eminent workers have told me that 

 theirs was similar. The common direct acting screw will 

 be found to be an inconveniently quick movement for photo- 



