380 



OBJECTIVES, EYE-PIECES, THE APERTOMETER 



piece is an under-corrected one and suitable for use with the 

 ordinary achromatic objectives ; by drawing out the sliding tube 

 and so increasing the distance between the eye and field lenses, the 

 so-called over-correction, which is associated with the compensating 

 eye-pieces, can be obtained in varying degree according to the 

 amount of extension. A scale is provided on the sliding draw- tube 

 for registering any desired position. 



There are theoretically two distinct advantages with this 

 eye-piece : 



(1) It obviates the necessity for being provided with both Huy- 

 ghenian and compensating eye-pieces, because it performs the 

 functions of both. 



(2) It will have been observed that with some objectives the 

 compensating eye-piece has appeared to possess too much over- 

 correction, producing the feeling in the mind of the worker that if 

 it were possible to vary the correction of the eye-piece a little a 

 better image could be produced ; this can theoretically be done with 

 the new ' Holoscopic ' eye-piece, but we prefer a definitely com- 

 pensated, or an ordinary eye-piece,. 



The initial magnifying powers of this series of eye-pieces are : 

 For the 160 nftn. tube length 5, 7, 10, and 14 diameters. 



250 7, 10, 14, 20 



For the English tube length, where the diameter of the eye-piece 

 fitting of the microscope permits of it, special!}' 

 large field-lenses are used. 



The cost is very little greater than that of the 

 ordinary Huygheiiian eye-pieces. 



The projection eye-piece is mainly intended 

 for photo- micrography, but it is also useful for 

 drawing and exhibition purposes. It is a negative, 

 with a single field-lens and a triple projection- 

 lens. The projection-lens is fitted with a spiral 

 focussing arrangement in order that the diaphragm 

 which limits the field may be focussed on to the 

 screen or paper. The field of this eye-piece is 

 small, but its definition is exquisitely sharp. 



It may not be generally known that good 

 photo-micrographs can be obtained by projection 

 with the ordinary compensating working eye- 

 pieces, but this is a fact worthy of note. 



It will perhaps be of practical utility if we 

 append a table indicating the focus of the com- 

 pensating eye-pieces when used with the long and 

 the short body. 



Special Eye-pieces. The most important of 

 these, the micrometer eye-piece, we have already 

 considered, so far as its application to micrometry is concerned. 1 

 Its optical character may be properly considered here. If it is a 

 negative eye-piece the micrometer is placed in the focus of the eye- 

 lens ; but if a positive combination, it is placed in the focus of the 

 eye-piece itself. The Ramsden form described above is thoroughly 



1 Chapter IV. 



FIG. 328. Zeiss's 

 projection eye- 

 piece No. 2. 



