PRELIMINAUY DIRECTIONS. 183 



the axis of the object-glass as possible. We now come to the 

 adjustment of the focus. 



Adjustment of the Focus. — If the focal distance of the object- 

 glass be known, then, without looking through the compound 

 body, it may be brought down within the focus, by turning 

 the large milled head of the coarse adjustment, and the eye 

 being applied to the eye-piece, the proper focus may be ob- 

 tained, by carefully turning the milled head backwards, until 

 either the object or some of the extraneous bodies generally 

 found upon the glass or in the fluid come into view. If the 

 power employed be a high one, it is always best to bring the 

 object-glass as near as possible to the focus with the coarse 

 adjustment, and to use the fine afterwards, when the eye is 

 applied to the eye-piece. If the object be not in the field of 

 view, then the glass or other apparatus on which the object is 

 placed is to be moved about by the stage adjustments untU it 

 appears there, or, in the absence of stage movements, the 

 fingers themselves can be employed. When the object is of 

 any tluckness, it will be readily found that, by the use of the 

 fine adjustment, any part, whether of the upper or of the 

 under surface, may be brought into focus ; and if it should be 

 transparent and cylindrical, like some of the vascular tissues 

 of plants, the markings on the vessels may be traced all roimd 

 the cylinder. In the instruments of Messrs. Smith and Beck, 

 one revolution of the milled head will either raise or depress 

 the object-glass about the -^^ of an inch, and being graduated 

 into ten divisions, the value of one division will be the t^s 

 of an inch, so that the thickness of any object can be very 

 well ascertained to this minute quantity ; a plan of measuring 

 that was first adopted by Mr. Valentine iu the microscope, 

 described at page 61. By this contrivance, also, certain pits 

 or depressions in any object can be accurately measured, by 

 bringing first the upper or plane surface into focus, and then 

 noting how many revolutions or parts of a revolution are gone 

 through before the bottom surface is equally well defined. 

 The graduation of the milled head will also be found of great 

 service for properly adjusting the front lens of the higher 

 power object-glasses, as the thickness of the thin glass cover 



