112 



PRACTICAL MICROSCOPY. 



In the use of these test objects, great attention must be 

 paid to the illumination and more particularly to the ad- 

 justment of the lenses for the thickness of the covering 

 glass. But little correction is needed with the screw collar 



FIG. 105. 



FIG. 106. 



adjustment in the water-immersion objective, and still less, 

 if any, is required with the homogeneous system ; never- 

 theless, it should always be added, as then correction is 

 possible, should it ever be required to meet exceptional 

 cases. In the lenses made by Messrs. Ross and Co., all the 

 powers from the ^--inch upwards can be used either as 

 ''dry " or "water-immersion," by merely moving the lenses 

 by means of the adjusting collar near to the mark "wet," 

 thus avoiding the cost of extra fronts and the inconvenience 

 in changing them. Great care is required in adjusting 

 them exactly, so as to get the best performance. 



3. FLATNESS OF FIELD AND FREEDOM FROM DIS- 

 TORTION. These properties in objectives may be tested 

 in several ways ; for low powers, a section of a large 

 spine of Echinus, such as shown in Fig. 107, may be used ; 

 a well cut and perfectly flat wood-section ; or perhaps 

 better still, one of Mr. Dancer's exquisite micro-photo- 

 graphs. The whole field should be well defined under 



