FOR THE MICROSCOPE. 



93 



tube (lo in. in the standard English instruments) by the nominal 

 length of focus of the objective. Thus : a i-inch objective mag- 

 nifies lo diameters, and if used with a 12 eye-piece the amplifica- 

 tion is 10X12 = 120 diameters. Similarly, a ^-inch objective 

 magnifies 40 diameters, and if used with an 18 eye-piece the total 

 amplification is 40 x 18= 720 diameters. 



Another very important advantage in the new system of con- 

 struction arises from the fact that, whereas in the old achromatic 

 arrangement the image formed by the lenses was so imperfect that 

 deep-eye piecing could not be resorted to without almost certainly 

 breaking down the image ; in the apochromatic combinations 

 amplification may be accomplished by the use of the deepest 

 oculars without at all impairing the definition. This enables the 

 worker to take advantage of the longer working distance, which 

 the focal length of the lower power objective gives him, and also 

 secures the greater amplification with less costly apparatus. 



The advantages of the apochromatic system may be conven- 

 iently summarised as follows : — 



In the Achrotnatic System. 



The sharpness of the image is 

 limited to 07ie colour of the light 

 rays — the green-yellow in ordi- 

 nary lenses; the blue-violet in 

 lenses for photographic pur- 

 poses ; the other rays giving 

 confused images. 



Complete colour correction is 

 effected in one zone only of the 

 objectives, the peripheral and 

 central zones showing a marked 

 imperfection. And only two 

 colours are united in one focus 

 in that zone. 



In the Apochromatic System. 



The images are very nearly 

 equally sharp for all colours of 

 the ■•spectrum, and therefore 

 scarcely affected by the colour 

 of the illuminating light. 



Colour correction is uniform 

 in all the zones, while thf'ee 

 colours are united in one focus. 

 By this means the amount of 

 focal difference for the various 

 colours of the spectrum is re- 

 duced to less than i/8th of its 

 original amount, which is a 

 practical elimination of the dif- 

 ferences. 



A considerably increased con- 

 centration of light, and the re- 



