THE WATER-IMMERSIUS OBJECTIVE 135 



sam, in which refractive differences of cytoplasm, chrom- 

 osomes, etc. are mostly lost, and vision depends mainly 

 on artificial color differences. But this method, which has 

 given such good results during the past 50 years, requires 

 to be supplemented; first by dark-field illumination; and 

 second by the observation in a bright field of objects not 

 resinified, showing all natural differences of refraction. 

 This latter is the function of the two improved high-power, 

 water-immersion objectives here mentioned. However, 

 the water-immersion objective should be mainly in the 

 hands of the more or less experienced worker, and should 

 not be entrusted to the beginner in microscopy. 



Practical Points 



1. The oil-immersion objective is superior for objects in 

 immersion oil or balsam, or for objects in optical contact with the 

 cover-glass. 



2. For objects deeper in water than a few microns, the water- 

 immersion objective is superior, as is seen in most iron-aceto- 

 carmine preparations. 



3. The cover-glass should be 0.17 millimeter thick, as closely 

 as possible, and should be measured by a gage; or, if on the shde, 

 by a dry objective and the micrometer screw of the microscope 

 (multiplying the measurement in microns by 1.5 in this case). 



4. The high-power, water-immersion objective should have a 

 correction collar. The collarless objective of one maker is 

 better corrected, the writer finds, by unscrewing than by increas- 

 ing the tube length, for thin covers. 



5. Water-immersion objectives should be used with distilled 

 water only, and the front lens must be kept free from traces of oil 

 or grease. 



6. If the high-power water-immersion does not resolve 

 Surirella gemma, mounted in hyrax, into (a sharp net or) sharp 

 dots, some adjustment is wrong, or there is oil or grease on the 

 front lens. 



7. If the adjustments are all correct, a high-power water- 

 inunersion objective will stand a ^{o condenser cone, without 

 glare, on a well-stained object. 



8. The water-immersion objective is specially suited for 

 preparations under large covers (20 by 50 millimeters or so) 



