320 Address by the President. [Sess. 



worked surfaces mounted between two thin metal plates by 

 which Leeuwenhoek made his magnificent discoveries, to the 

 splendid first-class microscopes manufactured by the modern 

 leading opticians. 



As you are all aware, there are two causes which prevent 

 the human eye seeing an object distinctly : these are, distance 

 and smallness. To overcome distance we employ an optical 

 instrument called a telescope, and to overcome the smallness 

 of an object we employ a microscope. A microscope is thus 

 an instrument to enable us to see objects which are almost or 

 altogether invisible to the naked eye. In practice we find 

 that there is a distance from the eye at which objects, accord- 

 ing to their size, are most clearly seen. This distance is called 

 the distance of distinct vision ; it, of course, varies in the case 

 of different persons, but for optical calculations it is usually 

 assumed as 10 inches or 250 mm. If we look at a page of 

 printed matter in which there are some lines of large type, 

 some of medium size, and some of small type, we find that we 

 can read the lines of large type at a greater distance than 

 those of the smaller type, and that the smaller the type the 

 nearer the eye the lines have to be brought. This shows that 

 the smaller the object, the nearer the eye it has to be brought 

 to enable it to be seen distinctly. There is, however, a limit 

 to this, for when the object is brought too near the eye it 

 again becomes indistinct. If, then, in this case a lens of a 

 particular form, say bi- convex, be interposed between the 

 object and the eye, it will bend some of the rays of light pro- 

 ceeding from the object from their original course, and bring 

 some of them together at such an angle that they will enter 

 the eye and thus enable the object to be seen distinctly. 



This power which is possessed by all transparent bodies, 

 whether solid or liquid, of bending from their original course 

 the rays of light passing through them, is termed the power 

 of refraction. There is another power, called the power of 

 dispersion, which transparent bodies possess of breaking up 

 and spreading out like a fan the primary colours of white 

 light passing through them. It is these properties of refrac- 

 tion and dispersion which lie at the foundation of the ' prin- 

 ciples on which microscopic lenses are formed, and on which - 

 they are dependent for their magnifying power. All known 



