CHAPTEK IY. 

 OPTICAL APPARATUS CONTINUED. 



The Condenser is almost of equal importance with the 

 objective, and certainly the best object glass cannot be expected 

 to work at its best without a good condenser properly used. 

 A non-achromatized object glass is so evidently useless that no 

 one is likely to be taken in by one, but perhaps unfortunately 

 a non-achromatic condenser is often found a very tolerable 

 makeshift for an achromatic one. At all events, those who 

 cannot afford to buy an achromatic condenser need not despair 

 of producing very fine work, though [perhaps the very finest 

 is beyond their reach. 



For the lowest powers usually employed in our work, how- 

 ever desirable a substage condenser may be, certain diffi- 

 culties of illumination preclude the use of a condenser, and a 

 bull's-eye must suffice. But for all objectives of one inch and 

 higher power we strongly recommend the use of a substage 

 condenser ; the bull's-eye also may be used if necessary. The 

 cheaper and commoner kinds of condenser are non-achromatic, 

 consisting usually of three elements and varying in angular 

 aperture from low figures up to the numerical aperture 1.4 as 

 made by Zeiss and others, the latter being of course oil immer- 

 sion condensers. For lower power work up to (say) the ordi- 

 nary four-tenths o. g. of about 90 deg. the front element should 

 be removed from the condenser ; this front is usually fitted 

 with a metal cap pierced with a very small hole for centering 

 purposes. Even the third or lowest element may be used alone 

 as a condenser, but as a rule it is better either to use the two 

 lower elements or to omit the condenser entirely. 



Achromatic Substage Condensers are now made by all 

 opticians and used, so far as we know, by all good micro- 

 scopists aiming at the best results of either observation ox 



