XXXIV INTRODUCTION. 



may occasionally be withdrawn for the purpose of 

 cleaning the glasses, which is best done with a piece 

 of soft wash-leather ; the lower end of the body (D) 

 has a screw, to which is attached the achromatic ob- 

 ject-glass (E). The object-glass consists of three 

 distinct achromatic lenses of equal foci, mounted in 

 brass cells, so that, one or more can be used at the 

 same time, according to the power required : for ex- 

 ample, if the whole of a large object is required to 

 be seen at one view, a single achromatic must be 

 employed, but if only a particular part is desired to 

 be examined with a high power, then two or three 

 must be used combined. The object to be examined 

 is placed on the concave piece of glass, which fits 

 the aperture of the stage (F), or else on a flat strip 

 of glass, which is retained in its proper position by 

 being placed beneath the brass fork (G)". The sil- 

 vered concave mirror (H) must be so arranged that 

 a strong light from the sky, or from a lamp or can- 

 dle placed about eighteen inches from the mirror, is 

 reflected through the axis of the microscope. 



Whenever I found a plant infested by an Aphis 

 which appeared to be the vastator, I secured speci- 

 mens of it in a pill-box, and in the evening placed 

 it in Canada balsam, and examined it more carefully- 

 In this way I have preserved all my evidences upon 

 this point for future reference ; and as the name of 

 the plant on which the insect fed was immediately 

 scratched on the glass with a diamond, no source of 

 error could possibly arise. 



