8 THE MICROSCOPE. 



general arrangement of its parts, and then to apply 

 the higher eye-pieces and powers, so that the more 

 minute structural details may be observed. 



Illumination. The illumination or proper manage- 

 ment of the light in using the microscope is of very 

 great importance. The best light, especially with the 

 low powers, is daylight, particularly that reflected 

 from white clouds ; this is least injurious to the eyes. 

 But as daylight cannot always be used for micro- 

 scopic investigations, and as cloud-light is insufficient 

 with the higher powers, some kind of artificial light 

 must be supplied. That mostly used is the light of 

 a reading oil lamp or of a gas-burner, a candle 

 being quite useless, on account of the flickering of the 

 flame with the slightest draught. A moderator lamp 

 has the defect of too great height; otherwise this 

 would be better than any other oil lamp. A power- 

 ful and excellent light for the highest powers is 

 afforded by a short paraffine-oil lamp. 



As intently looking at strongly illuminated objects 

 is injurious to the sight, the amount of light allowed to 

 pass the diaphragm should be no more than is agree- 

 able, and sufficient to show the object distinctly. 



In using the higher powers, the field is much less 

 bright with the use of the same light than in the case 

 of the lower powers ; and difficulty is often found in 

 obtaining sufficient light for the distinct vision of the 

 object. The achromatic condenser is of most im- 

 portant service here ; but it is sometimes requisite, 

 even when this is used, to condense the light upon 

 the mirror by a shallow bull's-eye ; or a large common 

 metallic reflector may be used for the same purpose. 



The most convenient manner of proceeding in re- 

 gard to the use of the individual eyes is to apply the 

 left eye to the eye-piece, so that the right eye may 

 be used in finding the stage-movements, or in moving 

 the slide, without removing the eye from the eye- 

 piece. If this arrangement be adopted, the light 



