23 



in importance to the objective, and a correct knowledge of its use is 

 necessary in order to obtain the best results from the objective. Stops 

 or diaphragms, or, better still, an iris diaphragm, should be fitted below 

 the condenser for the purpose of modifying the light transmitted. 

 The "Abbe" illuminator will be found a very useful form of con- 

 denser, and is moderate in price. It is composed of two lenses, the 

 upper one being removable for low power work. 



In the cheaper form of stand focussing of the objective is effected 

 by a sliding tube, but a rack and pinion movement is more con- 

 venient, and tends more to accuracy ; this is termed the " coarse " 

 adjustment, and, for preference, the diagonal form is recommended. 

 For those workers to whom expense is an object the sliding adjust- 

 ment is satisfactory as far as it goes, and it will save considerably on 

 the cost of the stand. For focussing the higher powers a reliable 

 " fine " adjustment is necessary. There are several forms of these 

 adopted by different makers. There is one which actuates move- 

 ment of the nosepiece at the end of the tube ; it is found in the 

 older type of stand, but is not to be recommended. The adjust- 

 ment now in general use is that which moves the entire body of the 

 microscope by means of a finely graduated micrometer screw. Care 

 is necessary in focussing so as not to injure lens or object by careless 

 use of the adjustment. Whenever focussing can be done with the 

 coarse adjustment, it should be used in preference to the fine adjust- 

 ment, the latter being a delicate piece of mechanism. 



The " stage " is that portion of the stand for the support of the 

 specimen to be examined, and is a perfectly flat metal plate rigidly- 

 fixed at exact right angles to the axis of the microscope tube. 

 Spring clips are fixed on it for holding the specimen. Some stages 

 have a rotary movement, which is occasionally useful ; others are 

 fitted with what is called a mechanical stage, which may be moved 

 in either direction by means of a rack and pinion. This adds con- 

 siderably to the cost of the stand, and for ordinary use it is unneces- 

 sary, but for work with the higher powers and the examination of 

 bacteria it is most essential. 



Proper illumination of the object is a matter requiring as much or 

 even greater care than the actual focussing of the objective. 



The " mirror " reflects light through the condenser on to the 

 object, and is of two kinds — concave and plane. The plane is 

 intended for use with the condenser for critical work with high 

 powers, the concave for general use with low powers without the 

 condenser. It is attached to a swinging rod, and should be capable 

 of movement in every direction. Daylight or artificial light may be 

 used ; for the latter a microscopical lamp with a ^-ir\. wick burning 

 paraffin is most suitable, and is, indeed, necessary when working 

 with high powers. A little camphor dissolved in the paraffin is of 

 advantage, as it produces a whiter light. 



The "bull's eye," or stand condenser, is a plano-convex lens of 

 about i \ in. to 2 or 3 in. diameter, and is used for concentrating 



