290 



ACCESSOKY APPARATUS 



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

 cipal focus does not 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 

 such a distance, but this is small. We can see, therefore, that this 

 error tends to slightly increase 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 

 by the initial power gives the multi- 

 plying power of the eye-piece. Care 

 must be of course taken to notice the 

 tube-length l 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 micros'cope 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-pieces. 



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

 our predecessors 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 fits 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 century. 



But in the early days of the microscope rotating discs of objec- 

 tives, as shown in fig. 228 (or, perhaps, older still, a long dovetailed 



FIG. 228. Kotating disc of 

 objectives. Benj. Martin 

 (circa 1776). 



FIG. 229. .Sliding plate of objectives. Adams (1771). 



slide of objectives, such as fig. 229 shows), were frequently 

 employed. 



It is continually desirable to be able to substitute one objective 

 for another with as little expenditure of time and trouble as possible, 

 so as to be able to examine under a higher magnifying power the 

 details of an object of which a general view has been obtained by 



1 English Mechanic, vol. xxxviii. No. 981, ' Optical Tube-length, by Frank Crisp. 



2 Ibid. vol. xlvi. No. 1178, ' Measurement of Power,' by^E. M. Nelson. 



