118 TRANSACTIONS OF THE 



or focus, which in most cases would not be visible to the unarmed 

 eye. Magnifying power has to do with si^e only, and refers to the 

 angle subtended by the enlarged image at the eye, as compared 

 with the angle subtended by the object itself under the circum- 

 stances of ordinary vision, which is about 10 inches from the eye. 

 The nearer an object is approached to our eyes, the larger it 

 appears ; and if our eyes could accommodate themselves to the 

 shortest distances, we would have no necessity for microscopes or 

 magnifying glasses. 



There are microscopes and microscopes, an infinite variety of 

 models, many of them constructed for special purposes. They are 

 all divided into two classes, namely, simple and compound micro- 

 scopes. The simple microscope may consist of one, two, or 

 three lenses, but these latter are so arranged as to have only the 

 effect of a single lens, i.e., they show the object itself magnified. 

 In the compound microscope not less than two lenses must be 

 employed, one to form an inverted image of the object, which 

 being nearest to it is called the object-glass, and the other to 

 magnify this image, and which is called the eye-glass, because it 

 is nearest to the eye of the observer. Both object-glass and 

 eye-glass, eye-piece or ocular, as it is commonly called, may be, 

 and in good microscopes always are, composed of a number of 

 lenses. 



Like most other inventions, the microscope has its history, but 

 time will only allow me to glance briefly over it. Simple micro- 

 scopes seem to have been in use thousands of years ago, as we 

 have not only ancient records of microscopic work having been 

 done, such as minute engravings, minute writings, and minute 

 mechanical contrivances, many of them contained in a nutshell — 

 hence the common expression, '^ in a nutshell," — which could not 

 have been made with the unassisted eyes, unless the visual powers 

 of former generations had been immensely superior to our own, 

 which was not the case ; but lenses of rock crystal, many of them 

 of short focus, have also been found, which were clearly intended 

 for optical purposes, and not as ornaments, as some have sought 

 to maintain. 



Zacharias Jansens and his son are said to have made microscopes 

 before the year 1590, one of which was at that time brought to 

 England. Fontana, in a work published in 1646, maintains 

 having made microscopes as early as 1618. It was with such 



