TESTS OF DEFINITION. 



109 



a deep eye-piece, unless the corrections have been well 

 cared for. 



The student should, if possible, compare this object under 

 two objectives, one of foreign make yielding an amplifica- 

 tion of 50 diameters with the A eye-piece, the other a i-inch 

 of English construction, possessing an air angle of 30. 



Another exceedingly good test of definition is a well 

 mounted specimen of the tracheal system of Dytiscus 

 marginalis, delineated in Fig. 100. 



In this instance the spiral threads should be visible with- 

 out any halo of colour, and clearly separated so as to appear 

 a continuous fibre 

 rolled between two 

 membranous walls. 



The engraving 

 shows the illusive ap- 

 pearance generally 

 perceived in this ob- 

 ject that of watered 

 silk produced by 

 the contact of the 

 back and front of the 

 spiral, at different in- 

 clinations, when the 

 object is flattened and 

 mounted in the usual 

 way in balsam. 



In order to discover how the correction for colour has 

 been performed, several objects may be employed ; Dr. 

 Carpenter's test is the section of pine-wood shown in Fig. 

 101 ; it should be mounted dry, and the small circles 

 (glandulcz) must be well defined and free from colour 

 even with the D eye-piece. Perhaps absolute freedom 

 from colour under deep eye-pieces does not yield the 



FIG. 100. 



