Lever Fine- adjustment. By E. B. Stringer. 297 



Mr. Nelson's horseshoe stage easily admits of this, and, by a 

 very simple contrivance, the top lens of the condenser is made to 

 swing aside, so that a well-corrected condenser adapted to the power 

 of the objective becomes at once available. The Continental turn- 

 out devices, as usually made, are wanting in this respect, since they 

 afford no variation in power, and the alternative of either a badly 

 corrected condenser or no condenser at all. 



Whilst speaking of Petrological Microscopes, it may be worth 

 while to draw attention to the great utility of the Bertrand lens in 

 ordinary work. Mr. Allan Dick has, indeed, in a measure already 

 done so. I think no one who has ever used it would willingly 

 dispense with it afterwards. To be able to bring the magnified 

 back lens of the objective into view with a touch of the finger 

 is a very great advantage. The centring of the condenser, the 

 extent to which the back lens is filled, and any diffraction spectra 

 which may be present, are seen as in no other way, and air-bubbles 

 in the immersion-oil are at once detected. It may even be used as 

 a finder, since, when the objective is above its focal point, a very 

 large area of the object is brought into view, imaged in the back 

 lens. In this way I have seldom failed to find the part of a slide 

 wliich I wanted A^athout substituting an objective of lower power. 

 The lens is, of course, also well adapted for use with the Aperto- 

 meter. 



I may add that the objective clutch fitted to the Leitz and 

 Eeichert Petrological Microscopes will be found an excellent device, 

 and quite as convenient for all purposes as a revolving nose-piece. 



June 19th, 1912 



