538 



Transactions of the Society. 



Fig. 145. 



Moreover the adjustment may be made without removing either the 

 flange or the eye-piece from the Microscope-tube ; and the stationary 

 pointer remains always in sight. 



The object of this adjustment is to ensure an invariable tube- 

 length, and not, as in the Huyghenian form, to maintain the correc- 

 tions of the eye-piece, which of course in this case remain unaffected. 



The lens may be easily re- 

 moved from the mount by un- 

 screwing the diaphragm in front 

 of it. The same mount m>ty 

 thus be used for either the over- 

 or under-corrected lens, and the 

 user need have but one mount 

 for both of them. 



The present lens is an over- 

 corrected one made specially for 

 me some time ago by Messrs. 

 Watson. 



I find that the Steinheil posi- 

 tive eye-piece is also particularly 

 adapted for visual work in con- 

 junction with a nicol prism. 

 The pencil issuing from it is so 

 slightly convergent, and the dis- 

 tance between it and the eye so 

 considerable, that the nicol may 

 be used above it, and even a 

 calcite plate as well, without 

 cutting off any of the field ; and 

 though this field is small, yet it 

 is much larger than can be got 

 with the nicol over the Huy- 

 ghenian eye-piece, where more 

 than halt' of it is lost. The 

 also of :auch greater bril- 



results obtained with the Steinheil are 

 liancy. 



The lens of this eye-piece is one of the ordinary Continental lowps 

 magnifying six times, and the nicol is mounted so that it may also be 

 used if necessary above the objective. It slips into the upper end of 

 the eye-piece where it may be freely rotated, and is supported by a 

 collar which maintains it at the proper distance from the lens. 



