18 HOW TO USE THE MICROSCOPE 



it has limited power, and one particularly objec- 

 tionable fault: its lens are not achromatic, or colour- 

 corrected. In consequence, objects viewed through 

 it tend to appear surrounded with a rainbow halo 

 inimical to critical work. The same fault is very 

 evident in the httle compound microscopes sold by 

 opticians for a few shilhngs. These cheap instru- 

 ments usually have two or three non-achromatic 

 " powers," which screw into each other, low mag- 

 nification being secured by use of one power, and 

 higher by screwing on one or two more. They are 

 too hghtly made for steadiness, and give httle stage 

 room for manipulation of objects. For the price at 

 which they are sold one could hardly expect any- 

 thing better, but no serious worker would consider 

 them for a moment. For good work a reliable 

 instrument must be purchased, and, taking every- 

 thing into consideration, a satisfactory microscope 

 may be obtained for a moderate figure, as we shall 

 now see. 



The intending purchaser of a microscope should 

 make the stand his first consideration, taking care 

 that it is firm on its foot, that its coarse and fine 

 adjustments work evenly and without jerkiness, and 

 that the working distance between the nose of the 

 body tube and stage is sufficient to allow of the use 

 of low-power objectives. The mirror should be 

 double, having a plane surface for low-power 

 illumination and a concave surface for high-power 

 work. A substage fitting is also a valuable ad- 

 junct. Having secured a good stand, objectives, 

 eye-pieces, and other accessories can be added 

 according to need and available cash. 



