F. IAN G. RAWLINS 189 



sketched in vertical section,) is inserted into the ring. The top is 

 provided with a slightly bevelled edge, into which fits a glass slip on 

 which the plasticine holding the specimen is placed as usual. This 

 can then be focussed upwards and downwards, avoiding any move- 

 ment of the body-tube. To substitute another specimen, all that 

 is needed is to rack down, swing the fitting out of the optic axis, 

 take out the plug, insert another levelled specimen as already 

 described, re-insert the plug, and focus as before. Of course, if 

 only objectives are being changed, the focus can be re-set at once. 

 An adapter fitted to the body-tube may be wanted if the rackwork 

 on the stand is limited. The central aperture in the stage is 

 generally too small, and should be enlarged for these additions. 

 In the event of transparent work with condenser, polariser, etc., 

 being contemplated, the focussing substage is ready at hand, the 

 appropriate fitting being substituted for the afore-mentioned plug. 



(2) Objectives. 



Mounting in short barrels is very desirable for use with the 

 vertical illuminator. There is often considerable difficulty in obtain- 

 ing objectives so arranged from the makers. The following alteration, 

 easily carried out, may assist. The lower part of the barrel is 





a 



Fig. 2. 



Carrier and Unscrewed Part of 

 Objective. 



unscrewed, and then inserted into a carrier bodily, which latter is 

 provided with a standard thread, attaching to the vertical illuminator, 

 and bringing the back lens of the objective very close to the reflector. 

 Two lenses so treated, a ^ inch and a ^ inch, in the writer's 

 possession give excellent results in practice. 



(3) Illumination. 



A type of " Half- Watt " lamp made in Holland has been found 

 admirable. The 200 candle-power size is amply sufficient. By noting 

 that the ring-filament in these lamps gives a very solid and concen- 

 trated area of light, and using a bull's-eye condenser of small aperture, 

 it is possible to get effects closely resembling a " point-source " 

 of light. The very moderate cost of these lamps compared with, 

 say, a " Pointolite," is greatly in their favour, and they are quite 

 powerful enough for magnifications up to 600 diameters in metallo- 

 graphy. 



In conclusion, apart from general ideas, the author disclaims 

 any question of having originated the above improvements. His 

 thanks are due to Messrs. H. F. Angus and Co. for their skilled 

 assistance. 



