4 I. THE COMPOUND MICROSCOPE STRUCTURE OF STARCH. 



The object is magnified by the objective, and the image thus 

 formed is again magnified by the eye-piece. With this instrument 

 are supplied two objectives, a low power of f inch (16 mm.) and 

 a high power of J (4 mm.) focal distance. 1 To obviate the 

 necessity, with every change of objective, of unscrewing the 

 one and replacing by the other, the microscope is provided with 

 a double nose-piece (N), seen in position in the figure. This is 

 screwed into the lower end of the tube, and into its two sleeves 

 are screwed the two objectives (o) and (o). By a slight lateral 

 pressure the nose-piece rotates on its centre, and one objective 

 replaces the other in position for use.- This nose-piece when 

 well constructed is accurately centred, so that the optic axis of 

 the one objective exactly replaces that of the other, and is tight 

 fitting, to exclude dust ; but if the microscope is not in use for 

 any length of time, it is better to unscrew the objectives, and 

 place them in the small brass boxes provided for them. One 

 eye-piece, No. I., is provided with the instrument, but it is better 

 to have also Nos. II. and III. (5s. each). This gives a linear 

 magnification without " draw-tube," ranging from 46 to 516 

 diameters, and a glance at the figures in this book will show 

 how rarely the latter magnification is exceeded. In general 

 No. I, should be used. The rough focussing (" coarse adjustment") 

 of the instrument upon the object is effected by means of the 

 sliding tube (T) which is pressed up or down in the sheath (SH), 

 this latter being lined with cloth to make the movement steadier. 

 Focussing should be performed very carefully, with a slight zigzag 

 twisting of the tube in the sheath, the left hand holding the 

 microscope steady by the fingers being placed on the shoulder of 

 the foot, and the right hand focussing, the second finger of the 

 hand being in constant contact with the tube lohere it enters the- 

 sheath, and with the top of the sheath itself, by which means the 

 rate of movement can be estimated and controlled. A glance at 

 the two objectives will show that the low power (j inch) has 

 the larger front lens, and the shorter setting or body. The 



1 The makers allow a choice of objectives ; this is the best combination. 



2 The double nose-piece must be used with care, especially if too much 

 mounting fluid be used, so that some flows out from under the edge of th& 

 cover-glass. With a thick fluid such as glycerine (so often used as a mountant) 

 the extruded drop may be so high as to come in contact with the front lens, 

 of the high power objective as it swings into or out of place, necessitating its. 

 removal for cleansing, and the attendant risks. 



