332 Address by the President. [Sess. 



the form of a microscope. Microscopes are of two kinds, 

 simple and compound. The fundamental difference between 

 them is, that in the simple microscope the rays of light are 

 received by the eye direct from the object itself, of which a 

 virtual image is formed, as shown in Fig. 2, Plate XXVII. ; 

 while in the compound microscope the rays are received by 

 the eye, not from the object, but from a real image of it. 



The Simple Microscope. — The principal use of the simple 

 microscope is for the purposes of dissection. The virtual 

 image of the object which it forms is not inverted, so that 

 dissecting can be easily carried on under it. At first the lens 

 was but a single double-convex one ; afterwards Dr Wollaston 

 added another lens, when the combination was called a doublet. 

 Mr Holland suggested a third lens, and thus the name triplet 

 was applied. One of the best forms of these lenses is that 

 known as the Steinheil lens. It is achromatic, and gives a 

 large flat field of view. A holder of some kind is necessary 

 for the lens, so that both hands may be free. There are 

 various forms of dissecting microscope on the market of more 

 or less completeness, but with a little ingenuity a very service- 

 able one could be made at very small expense, using an ordin- 

 ary pocket-magnifier as a lens. 



The Compound Microscope. — The compound microscope in its 

 simplest form is a combination of Figs. 1 and 2, Plate XXVIL, 

 — that is, one set of lenses, called the object-glass, of short 

 focus, forms a real image of the object at some distance behind 

 them, and this real image is caught up by another lens, called 

 the eye-glass, which forms a virtual image of the same, and it 

 is this virtual image which is seen by the eye on looking into 

 the microscope. We have seen how the object-glass is con- 

 structed, and need not go back upon it. Its function is to 

 form an enlarged real image of the object, and its position is 

 thus near to the object. The eye-glass is that to which we 

 apply the eye. There are several different kinds, but the 

 form most commonly in use is called a Huyghenian, from 

 Huyghens, a Dutch astronomer, who devised it. This form 

 consists of two plano-convex lenses, fitted one at each end of 

 a short brass tube, with a diaphragm between them to cut off 

 extraneous rays. The lens to which the eye is applied is 

 called the eye-glass, and the other lens is called the field-glass, 



