particular spot desired, for if this spot is in the center of 

 the field of the low power it will be somewhere in the 

 field of the higher power. It is too much to ask of the 

 maker that the lenses be made absolutely parfocal and 

 centered. The delicacy of the centering can be appre- 

 ciated when the magnification and the extremely small 

 portion examined is considered. When the objectives 

 are not thus fitted to the nosepiece, re-focusing and 

 again hunting up the object are necessary. In doing so 

 we repeat the caution to always focus up before turning 

 the nosepiece. When no revolving nosepiece is used 

 the change of objectives .means the unscrewing of one 

 and the screwing of the other into its place, and re- 

 focusing as before. 



ILLUMINATION WITHOUT SUBSTAGE CONDENSER. 



Central Light. It has been necessary in the fore- 

 going paragraphs to secure some light upon the specimen, 

 but no directions have been given as to the proper 

 illumination of the same. Accuracy of results depends 

 upon correct illumination more than any other one 

 thing. A vast majority of all microscopic work is done 

 by light transmitted through transparent or semi-trans- 

 parent objects. We will at present consider only such 

 objects. The matter of illuminating opaque objects will 

 be taken up later. The mirror is placed below the stage 

 as a convenient means of reflecting the light through 

 the object into the objective. It is plane on one side and 

 concave on the other. The concave mirror is always 

 used when the substage condenser is not used, except in 

 the case of very low power objectives, when it is best to 

 use the plane mirror. 



When the light is thrown upon the specimen and the 

 objective focused as previously directed, remove the 

 ocular and look into the tube at the back lens of the 

 16 



