GAIRDNER S DOUBLET MICROSCOPE. 



49 



through the film of liquid ; and when the focal adjustment is 

 made by means of the milled-head, any particles it may contain of 

 a size to be brought into view „ „ 7 



by the magnifying power em- 

 ployed, will be distinctly dis- 

 cerned. The instrument is 

 usually constructed with but 

 a single power, adapted to the 

 class of objects for which it is 

 to be employed ; thus for the 

 purposes of the Botanical or 

 Zoological collector, a power of 

 from 70 to 100 diameters is 

 sufficient ; whilst for the exa- 

 mination of Urinary deposits, 

 a power of 200 or more is 

 desirable. It would not be 

 difficult so to modify it, how- 

 ever, by making the Doublet 

 to screw into a socket, instead 

 of fixing it on the stem, that 

 one power might be substi- 

 tuted for another on the same 

 instrument; and the adjust- [<D< 



ing screw might then perhaps 

 be dispensed with, since the 

 focal adjustment might pro- 

 bably be made sufficiently 

 well, by turning round the 

 doublet itself in its screwed 

 socket. The object-holder, 

 too, might be so constructed 

 as to receive a greater variety 

 of objects, and even to hold preparations mounted on slips of 

 glass, which would often be a matter of great convenience for 

 Class-demonstration. All this, however, would add to the com- 

 plexity and the cost of the instrument ; the simplicity and low 

 price of which at present constitute its chief recommenda- 

 tion. Though not suited for the higher purposes of a Microscope 

 (the view of any object afforded by a Doublet magnifying 100 or 

 200 diameters being far inferior to that presented by only a toler- 

 able Achromatic), yet there is a certain class of observations for 

 which it is particularly convenient — those, namely, which only 

 require a recognition of known forms. Thus, the Collector of 

 Diatoms, Animalcules, &c, may by its means at once test the 

 general value of the sample he has taken up, and may decide 

 whether to throw it away as worthless, or to reserve it for more 

 minute examination. And the Medical Practitioner who is familiar 



Gairdner's Doublet Microscope. 



