MOUNTING OF MICROSCOPES. 



76. The attitude of an observer stooping the head to view an 

 object in a microscope, whose eye-piece is vertical, is found to be 

 attended with much inconvenience, especially if the observation 

 be long continued. This has constituted the ground of a very 

 general objection to vertical microscopes. Nevertheless there are 

 many cases in which it would be inconvenient to place the stage 

 in an inclined or vertical position, as, for example, when observa- 

 tions are made on liquids. In all such cases the model of 

 Amici's stand presents obvious advantages, the observer looking 

 horizontally, while the axis of the object-piece is vertical, and 

 consequently the stage horizontal. 



Most of the better class of instruments, however, are so 

 mounted that any direction whatever can be given to the axis of 

 the body. Various mechanical expedients are used for accom- 

 plishing this, most of which are analogous to the methods of 

 mounting telescopes. In some, the instrument with its appendages 

 is supported upon two uprights of equal height by means of 

 trunnions, which pass through its centre of gravity, so that it 

 turns upon its supports like a transit instrument, the axis of the 

 body being capable of assuming any inclination to the vertical. 

 The observer, therefore, may at pleasure look obliquely or verti- 

 cally downwards, or obliquely upwards, as may suit his purpose. 



Similar motions are also produced by mounting the instrument 

 upon a single pillar by means either of a cradle-joint, such as is 

 generally used for telescope-stands, or a ball and socket. Stands 

 of this form are attended with the advantages of offering greater 

 facility for moving the instrument horizontally round its axis. 



In the attainment of all these objects, as well as in the produc- 

 tion of eye-pieces and object-pieces of capital excellence, the 

 leading makers of London, Paris, Berlin, and Yienna, have 

 honourably rivalled each other, and it may be most truly said, to 

 their credit, that if some have excelled others in particular parts 

 of the instrument, there is not one who has not in some way or 

 other contributed by invention or contrivance to the perfection 

 either of the optical or mechanical parts. 



Much however is also due to the eminent philosophers and 

 professors who have more especially devoted their attention to 

 those parts of science in which the microscope is a necessary 

 means of observation, and foremost among these is the patriarch 

 of optical science, Sir David Brewster. It would be difficult to 

 name the part of the instrument, or of its accessories or append- 

 ages, for the improvement of which we are not deeply indebted 

 to this eminent man. Among the more recent philosophers who 

 have contributed to the advancement of micrography, and by 

 whose researches and suggestions the makers have been guided, 



71 



