POSITION OF OBSERVER. 291 



error; when such illumination is employed the chances 

 are that all fine 'detail of structure will be drowned 

 out, while the effect on the eyes is most injurious. Keep 

 then the field just as cool as shall be consistent with 

 vivacity and life, show your objects brilliantly, and see 

 that even the shadows are lively and transparent. 

 Grasp the mirror firmly; get in the habit of this; 

 when you touch it at all take hold of it boldly. There 

 is considerable for the student to learn in the proper 

 handling of the mirror, so as to combine boldness with 

 delicacy and efficiency. Now as you change its posi- 

 tion just a trifle, notice the action of the light. Does 

 it slide by as it were, without taking hold? Or is there 

 one particular position in which the light seems to 

 catch and " nip," bringing out the makings with a vim. 

 Now if the former is the case, there is something wrong 

 (the objective being in adjustment) and perchance the 

 whole system of illumination will require attention in 

 detail. 



When the smallest shell of the Monmouth and 

 the Cherry field have been thus mastered (as to their 

 transverse striae) those of the Leipsig may be taken in 

 hand, and subsequently the smallest valves of the Isle 

 of Shoals; these are quite as severe tests as an ordi- 

 nary balsamed amphipleura pellucida. The slide of 

 Leipsig ought to be thoroughly mastered previous to 

 attacking the Isle of Shoals; all the peculiar appear- 

 ances which we have from time to time set forth, are 

 to be studied, and as the eyes become educated the 

 amount of obliquity of the illumination is to be de- 



