252 THE USE OF THE MICROSCOPE 



12. Uncorrected Condenser with Ground Glass in Front 

 Focal Level. — A piece of doubly ground glass (colorless 

 or bluish) is put in the diaphragm carrier of the uncorrected 

 condenser. A small 6-volt lamp is put close below, or a 

 strong light is focused on the ground glass by a large con- 

 densing lens, using the plane mirror. The condenser is 

 raised close to the slide. For water and oil-immersion 

 objectives, the condenser may be immersed. Test the 

 condenser light circle on the back of a high-power objective. 

 Note that it can be cut down as usual by the iris of the 

 condenser. However, the source is not cut down, and so 

 there is glare at high apertures. (Hartridge, Zeiss.) 



13. Dry Condenser Corrected by Unscrewing Top 

 Lens. — In a dry achromatic condenser, the upper lens is 

 usually a hemispherical single lens, while the two lower ones 

 are cemented doublets. Hence, by slightly unscrewing the 

 top lens, the combination can be corrected to suit a some- 

 what closer lamp or a thinner slide. Take an achromatic 

 condenser corrected for a lamp at 25 centimeters and for 

 a slide 1.3 millimeters thick. Unscrew the top lens suffi- 

 ciently to correct it for slides 1.0 millimeter thick. Test 

 the corrections, with a 3-millimeter diaphragm on the 

 source, by the disappearance or thinning out of the marginal 

 ring on the back of the objective on focusing the condenser 

 either down or up. (Spitta, Coles.) 



14. Correcting an Oil-immersion Condenser for Water 

 Immersion. — Take an oil-immersion condenser of about 

 8.5 millimeters focal length, corrected for parallel rays and 

 slides 1.2 millimeter thick. Substitute water for oil, with a 

 slide 1.0 millimeter thick, and a lamp distance of about 

 25 centimeters. Cancel the resulting undercorrection by 

 putting a supplementary achromatic lens, of about 15 

 centimeters focal length, and 30 millimeters across, close 

 below the iris of the condenser (centered in the diaphragm 

 carrier). Get the final adjustment by the ring test, altering 

 the lamp distance till all is correct. (Hartridge, Coles.) 



If the maker, however, has corrected the condenser for a 

 lamp at 25 centimeters, a much weaker accessory lens is 



