286 HOW TO SEE WITH THE MICROSCOPE. 



near closed with water as the intermedium, then, of 

 course, use water; or it may occur that the cover is an 

 exceedingly thin one so thin that even with the glyce- 

 rine the correction would obtain at some considerable 

 distance from closed, in which case select another very 

 thin cover to supplement that on the mount. All this 

 has been discussed before, but in the preceding lessons 

 one might have got along tolerably well without par- 

 ticular observance of these conditions, but not so with 

 the problems we are now about to tackle. 



Having then decided the above point make the im- 

 mersion contact, and selecting a medium shell of the 

 rhomboides, bringing it to the center of the field focus. 

 Arrange the mirror so as to get the strongest illumina- 

 tion ; let the lamp burn with a moderate flame ; if now 

 there is obviously too much light, a perfect glare, 

 bring the lamp closer to you (keeping the flame edge- 

 wise to the mirror constantly) moving the same in a 

 line parallel to the left hand edge of the stage, readjust 

 the condenser and mirror thus lighting up the field 

 again. It will be seen that the farther the lamp shall 

 be thus moved the less will be the illumination. The 

 point to stop at is when you have shut out the super- 

 fluous sheen or glare. All this time \ve have supposed 

 that the flat face of the condenser was parallel to the 

 face of the mirror. Now change the position of the 

 condenser causing it to assume a slightly diagonal pose, 

 the nearer edge to be swung away from the stage; 

 when the condenser is exactly right the mirror will 

 have the appearance of being traversed by a line ol 



