290 



ACCESSORY APPARATUS 



the error introduced by M small displacement of the posterior prin- 

 cipal torn- d<>e> lint materially amount to much. 



There is a further error introduced by the approximation of the 

 objective to the stage micrometer in order to focus the conjugate at 

 Mich a distance, but this i> small. We can see, therefore, that this 

 error tend> to .-lightly increa>e the initial magnifying power. 



The initial power of the J being 

 found, and its combined magnifying 

 power, with a given eye-piece, being 

 known, the combined power divided 

 li\ the initial power gives the multi- 

 plying power of the eye-piece. Care 

 must be of course taken to notice the 

 tube-length ' when the combined power 

 is measured. The initial power of any 

 other lens may be found by dividing 

 the combined power of that lens with 

 the eye-piece, whose multiplying power 

 has been determined, by the multiplying 

 power of that eye-piece. 2 



Nose-pieces. The term ' nose-piece' 



primarily means that part of a microscope into which the objective 

 screws, but the term is also applied to various pieces of apparatus 

 which can be fitted between the nose-piece of the microscope and 

 the objective. There are, for instance, rotating, calotte, centring, 

 changing, and analysing nose-piece^. 



Nose-pieces, although thought to be so. are not a modern idea : 

 our predeces.-ors of a century ago employed similar means. Mr. 

 Crisp has recently acquired a microscope which possesses a double 

 arm. at the end of which is a cell for receiving different lenses. 

 This cell fils over the end of the nose-piece, and so keeps the several 

 objectives which may be inserted in position. It dates, in all proba- 

 bility, from the end of the seventeenth or the early part of the 

 eighteenth cent ury. 



Hut in the early davs of the microscope rotating discs of objec- 

 tives. ,-is shoun in fig. -Ji!H (or. perhaps, older still, a long dovetailed 



FIG. 228. Rotating disc of 

 objectives. Benj. Martin 

 (circa 1776). 



J-J'.i. Sliding plate of objectives. Aduins (1771). 



slide of objectives, such as tig. 'J-J 1 .) shows), were frequently 

 employed. 



It is continually desirable to be able to substitute one objective 



for aiiot her wit h as little expenditure of t hue and trouble as possible, 



to be able to examine under a higher magnifying power the 



details of .-in object ol'whicli a general view lias been obtained by 



' M< <>1. '.xxviii. No. 981, ' Optical Till.. I, Mjih. 1,\ I'Yunk 



' 178, Measuri I ' m -.' by ( E. M. Nelson. 



