6 4 



BOTANY FOR BEGINNERS 



CHAP, 



One of the best and cheapest for botanical work is shown in 

 Fig. 83. It is called a triplet, because there are three lenses 

 mounted so that each one can be used by itself, or in combina- 

 tion with the others. To use such a lens to view a transparent 

 object it is necessary to place the lens close to the eye and to move 

 the specimen about until it appears bright and clear. The 

 object is said to be in focus when it is best seen. If the 

 specimen to be examined is opaque, the best way to observe it is 

 to move both the lens and object until a good view is obtained. 

 Transparent objects can be seen best with all the three lenses as 



Fig. 83. Diagram illus- 

 trating hand - lens, 

 i = low power. 2 = 

 medium power. 3 = 

 high power. 



FIG. 84. 



Diagram 



showing 

 position of 

 lens when 

 the highest 



power is 

 used. 



Diagram show- 

 ing position of 



lens when 



medium power 



is used. 



FIG. 86. Diagram illus 

 trating how to focus a 

 hand-lens. A, the dis- 

 tance of the object 

 from the lens when in 

 focus with the low 

 power ; B, with me- 

 dium ; and C, with 

 high power. 



shown in Fig. 84, but if the objects are opaque, with either lenses 

 i or 2, or i and 2 combined, as in Fig. 85. In Fig. 86, the 

 edges of the lenses are shown, and A, B, and C, indicates the 

 relative distances at which a specimen may be viewed by i, i 

 and 2 and i, 2, 3, respectively. 



EXPT. 52. Place a little cotton wool between two microscope 

 slips, and with the hand-lens find out the position in which it has to 

 be held so as to focus it with (i) the lower power, (ii) the medium 

 power, (iii) the highest power. 



