GAIRDNER'S AND FIELD'S SIMPLE MICROSCOPES. 93 



be difficult so to modify it, however, by making the doublet to 

 screw into a socket, instead of fixing it on the stem, that one 

 power might be substituted for another on the same instrument; 

 and the adjusting screw might then perhaps be dispensed with, 

 since the focal adjustment might probably 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 con- 

 venience for class demonstration. All this, however, would add 

 to the complexity and the cost of the instrument; the simplicity 

 and low price of which at present constitute its chief recommen- 

 dation. Though not suited for the higher purposes of a Micro- 

 scope (the view of any object afforded by a doublet magnifying 

 100 or 200 diameters, being far inferior to that presented by only 

 a tolerable 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 Diatomacese, 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 with the aspect of Urinary deposits, may, by this little 

 instrument (which he can carry in his waistcoat-pocket), discrimi- 

 nate on the spot the nature of almost any sediment whose charac- 

 ter he may wish to know, without being obliged to have recourse 

 to a more elaborate apparatus. 1 



29. Field's Simple Microscope. The general purposes of a sim- 

 ple Microscope are satisfactorily answered by the instrument, 

 which has recently gained the premium awarded by the Council 

 of the Society of Arts, and which is capable of being very effec- 

 tively used in the examination of most of the objects for which 

 such an instrument is suited. It consists (Fig. 16) of a tubular 

 stem, about five inches high, the lower end of which screws 

 firmly into the lid of the box wherein the instrument is packed 

 when not in use. To the upper end of this stem, the stage is 

 firmly fixed; while the lower end carries a concave mirror. 

 Within the tubular stem is a round pillar, having a rack cut into 

 it, against which a pinion works that is turned by a milled head ; 

 and the upper part of this pillar carries a horizontal arm which 

 bears the lenses ; so that, by turning the milled head, the arm 

 may be raised or lowered, and the requisite focal adjustment ob- 

 tained. Three magnifiers are supplied with this instrument ; and 

 by using them either separately or in combination (the lens of 

 shortest focus being placed at the bottom, whenever two, or 

 all three, are used together), a considerable range of powers, 



1 This Microscope, the invention of Dr. William Gairdner, of Edinburgh, is made by 

 Mr. Bryson, optician, of that city. 



