



26 Transactions of the [Sess. 



that all who care may come together of an evening, or at any- 

 other time, and give mutual help in the examination and perma- 

 nent preservation of special objects. With such advantages and 

 opportunities in the immediate future, we may surely expect that 

 our prosperity and influence will enormously increase. 



[The Chairman then indicated to the members lines of study and 

 investigation which might profitably be engaged in, and closed by 

 inviting them to work vigorously together for the advancement of 

 the Society.] 



At the second meeting (Dec. 4, 1885), Mr William Forgan made 

 the following valuable observations on microscope objectives : — - 



NOTES ON MICROSCOPE OBJECTIVES. 



Every one who uses a microscope should know a good deal about 

 the construction of the instrument, particularly the optical part. 

 This can readily be learned from the ordinary text-books, but 

 there are many points of importance in connection with the subject 

 which are apt to be overlooked unless the reader's attention is 

 directed to them in a special way. In the following notes, space 

 will not permit of a great deal being said. I can only refer to a 

 few of the more prominent and striking facts. 



Microscopes are usually divided into two classes — viz., simple 

 and compound. The simple microscope is that in which the 

 magnifying power is obtained by the use of a single lens, or, it 

 may be, two or three lenses placed together — the eye being placed 

 immediately behind them. I need not here enter upon the ques- 

 tion of how an object is apparently enlarged by viewing it through 

 a lens of short focus. That is explained fully in every book on 

 the microscope. I need only say that the enlargement arises from 

 the fact, that by means of the lens the object is viewed at a greater 

 angle, and hence appears larger to the eye. The simple micro- 

 scope remained for a long time without much improvement. 

 Lenses had been made of precious stones, which, having greater 

 density than glass, allowed the use of shallower curves with the 

 same magnifying power. It was only, however, when Dr Wollas- 

 ton invented his doublet that any advance was made. This en- 

 abled them to pass a larger angular pencil of light through the 

 objective for the simple microscope, and hence gave much clearer 

 and brighter definition of the object. Simple microscopes have 

 one quality which renders them useful where low powers only are 

 required — they do not reverse the image. Their use nowadays 

 is very limited, as the recent gigantic strides in the improvement 

 of the compound microscope have quite superseded them. The com- 

 pound microscope, previous to the invention of the principle of 



