XXV111 INTRODUCTION. 



removing from one instrument to the other. To this may 

 be added perhaps another advantage, in that the working- 

 table is not cumbered with an extra instrument. For dis- 

 secting with the microscope the wrists must be supported on 

 a level with the object, or slightly below it. Some dissecting 

 microscopes have arm-rests for this purpose ; blocks of wood 

 of suitable height, or even stacks of books, will answer 

 admirably. 



A very necessary adjunct for microscopical work is a good 

 magnifying lens, as it is often desirable first of all to study 

 an object with it, afterwards using the microscope. The 

 lenses of the dissecting microscope can be used as hand 

 magnifiers, and low-power objectives likewise make good 

 hand lenses. It is worth while, however, to get a lens mag- 

 nifying about six diameters ; very convenient are the triplets, 

 three lenses in a tortoise-shell case, usable separately or 

 together, and sold at a price of about 3s. 6d. Kemarkably 

 beautiful are the Platyscopic Lenses of Browning (63 Strand, 

 London, W.C.), magnifying fifteen, twenty, or thirty dia- 

 meters, price 15s. each, and the Aplanatic Lenses of Zeiss, 

 magnifying six, ten, or twenty diameters, price, 12s. or 15s. 

 each. Becks', also, are now manufacturing some new hand 

 lenses of very high quality at 11s. 6d. each. 



As it is desirable that the student should from the first 

 begin to draw the objects he examines (practical instruction 

 in which will be found on p. 40 et seq.}, it is desirable that he 

 should have some form of drawing instrument to facilitate his 

 work. Drawing can, it is true, be done without any such 

 aid, but is more difficult. An apparatus for drawing (camera 

 lucida) is constructed either for use with the body of the 

 microscope placed horizontally, or placed vertically. Practi- 

 cally the latter is much to be preferred. Every microscope 

 maker has appliances of his own make, but they vary very 

 much in real utility. Two typical instruments, both manu- 

 factured by Zeiss, are figured on pp. 40 and 42, and their 

 method of use is described. 



A stage micrometer is likewise necessary to enable measure- 

 ments of microscopic objects. This can be obtained from most 

 opticians at a cost of from 5s. to 10s., and ruled up to T oVo of 



