ADJOURNED DISCUSSION IN LONDON 235 



-could be easily slipped on and off. The under-stage tube was fixed 

 to a plate; this could be replaced by a centering motion, rack and 

 pinion sub-stage. The sliding draw-tube could be replaced with a 

 rack and pinion draw-tube divided into mm. 



Another addition which is added to a small portable microscope, 

 and would be useful to all plain stage microscopes, whoever the maker 

 might be, is a very simple and efficient finder, and is standardised 

 as follows : — 



Each maker has a 3 x 1 in. piece of metal 1 mm. thick. At 

 equal distances from the ends and sides is made a small hole (A) 

 1 mm. diameter, and another hole the same size, made exactly 1 in. 

 distance from the centre hole and equal distance from the sides (B), 

 thus : — 



When an instrument is assembled and completed ready for sale 

 the above plate is placed on the stage of the microscope resting 

 against the sliding bar or mechanical stage, or a mechanical square; 

 then with a 2 /3rd or 1 in. objective the hole A. is brought into 

 the centre of the field of the eye-piece; the metal 3 x 1 is held 

 firmly by the stage springs or clips, and a small sharp drill is passed 

 through the hole B and a few twists given, which will make a 

 drill mark on the stage. This is then filled in with Plaster of Paris, 

 thus giving a white dot over a black stage. 



Now suppose we have a scattered slide, and some part (or parts) 

 has some object of special interest which one wishes to find quickly 

 at some future time — all that is needed when the object is squarely 

 •on the stage is to make an ink dot on the slide exactly over the 

 white dot on the stage. Other dots can be made if needed, and 

 marked A, B, C, etc. Then for the future all that is necessary is 

 to place dot A, B, or C over the white dot on the stage, and the 

 •desired part is in the centre of the field of the eye-piece. 



With regard to Dr. Evans's paper, he has specially mentioned 

 •crystals, but I do not think any instrument is so efficient for examin- 

 ing minute crystals as the one introduced by Mr. Allan B. Dick. 

 In this instrument you can introduce a minute crystal on the cross 

 ■wire, and it does not alter its position at all. 



Dr. J. W. Evans : No one appreciates more than I do its valu- 

 able qualities, but it is impossible to apply the methods devised by 

 Professor Beck for the study of interference figures to a microscope 

 ■with rotating nicols, at any rate without very considerable modifica- 

 tion, and in the second place the small upper Bertrand lens cannot 

 compare in convenience and effectiveness for the examination of the 

 interference figures of minute objects with a Beck lens placed above 

 the eye-piece, in conjunction with a diaphragm placed in the focua 

 of the latter. 



