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CHAPTER IV 



ACCESSORY APPARATUS 



THIS chapter on apparatus accessory to the microscope might be 

 easily made to occupy the whole of the space we propose to devote 

 to the entire remainder of the book ; the ingenuity of successive 

 microscopists, and the variety of conditions presented by successive 

 improvements in the microscope itself, have given origin to a 

 variety of appliances and accessory apparatus that it would be futile 

 in a practical handbook to attempt to figure and describe. We pro- 

 pose, therefore, only to describe, and to explain the mode of success- 

 fully employing, the essential and the best accessories now in use, 

 neglecting, or only incidentally referring to, those which are either 

 supplanted, or which present modifications either not important in 

 themselves or accounted for by the fact of their production by 

 different opticians. 



I. Micrometers and Methods of Measuring Minute Objects. It 

 is of the utmost importance to be able with accuracy, and as much 

 simplicity as possible, to measure the objects or parts of objects that 

 are visible to us through the microscope. 



The simplest mode of doing this is to project the magnified 

 image of the object by any of the methods described under 

 ' Camera Lucida and Drawing.' We carefully trace an outline of 

 the image, and then, without disturbing any of the arrangements, 

 remove the object from the stage, and replace it with a ' stage micro- 

 meter,' which is simply a slip of thin glass ruled to any desired scale, 

 such as tenths, hundredths, thousandths of an inch and even less. 

 Trace now the projected image of this upon the same paper, and the 

 means are at once before us for making a comparison between the 

 object and a known scale, both being magnified to the same extent. 

 The amount of magnification in no way affects the problem. Thus, 

 if the drawn picture of a certain object exactly fills the interval 

 between the drawing representing the '01 inch, the object measures 

 the '01 inch, and whether we are employing a magnifying power of 

 a hundred or a thousand diameters is not a factor that enters into 

 our determination of the size of the object. In fact, all drawings of 

 microscopic objects are rendered much more practically valuable by 

 having the magnified scale placed beneath them, so that measure- 

 ments may at any time be made. 



In favour of the above method of micro-measurement, it will be 

 noted (1) that no extra apparatus is required, (2) that it is extremely 

 simple, and (3) that it is accurate. 



