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CHAPTER IV 



ACCESSORY APPARATUS 



THIS c-h.-i pter on apparatus accessory to the microscope might be 

 ea>ilv ma ile to occupy the whole of the space we propose to devote 

 tu 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 opt Lcians. 



I. Micrometers and Methods of Measuring Minute Objects. It 

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

 simplicity as p<sible, 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 l:n/>ir lt nc,il<>. 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 ()! inch, the object measures 

 'he -III inch, and whether \\eareemployinga magnifying power of 

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

 our determination of t he si/.e of 1 he 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- 

 ment v may at any t hue he made. 



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



d (!) t hat im extra apparatus is required. ('I) that it is ext remely 

 simple, and (.'!) that it is accurate. 



