336 SUMMARY OF CURRENT RESEARCHES RELATING TO 



tube type, where the eye-lens is moved to focus it to the scale. With 

 the focusing scale the power of the eye-piece, therefore, remains con- 

 stant. An observer with - 4-D. of presbyopia now compares the 

 magnified image of the stage micrometer with the lines of the scale, 

 and finds that each of the large divisions in the scale is equal to 84 micra. 



Another observer, with + 4*0 D. of myopia, now makes the same 

 comparison, but finds that the large division is equivalent to 91 • 4 micra. 



Now, at first, one would think that the myopic observer was using a 

 Microscope of lower power, because he had to get a larger interval in 

 the stage micrometer, or, in other words, a larger object to fill the same 

 space in the eye-piece scale. But a little reflection will show that it 

 is owing to the extra power of the myopic eye making the eye-piece 

 scale larger that a larger object is required to fill it. Abbe asserted 

 that with a simple Microscope the magnification is the same for both 

 kinds of vision, but the above experiment shows that this is not the 

 case. With micrometer eye-pieces of the usual form, i.e. with focusing 

 eye-lenses, the reading with ]>oth kinds of eyesight will be the same, 

 because the magnification eff'ect on the scale will be proportional to 

 that on the Microscope as a whole ; but when the scale only is focused, 

 the power of the Microscope as a whole remains practically constant, 

 while that of the eye-lens of the eye-piece undergoes considerable change. 



Evans, J. W. — The Determination of Minerals under the Microscope by means of 

 their Optical Characters. 



[This is a full and exhaustive treatise upon the subject.] 



Journ. Quekett Micr. Club, xii. (1915) pp. 597-630 (3 pis.). 



(6) l\Iiscellaueous. 



Optical Glass : an Historical Note.* — F. J. Cheshire writes to 

 Nature " as follows : — The subject of optical glass is, at the present time, 

 one of such paramount importance that no apology is needed for intro- 

 ducing it to the attention of your readers. As is well known, the Rev. 

 Vernon Harcourt and Sir (leorge Gabriel Stokes, in the earlier half of 

 last century, laboured together for more than twenty-five years with the 

 object of adding to our stock new varieties of optical glass, but without 

 success. Their labours, however, were afterwards continued by Abbe 

 and Schott, of Jena, who, in the course of some five years, were com- 

 pletely successful. As the result of a critical examination of the work 

 of the English workers, Czapski — then the head of the firm of Carl 

 Zeiss, of Jena — came to the conclusion that Harcourt and Stokes had 

 failed simply because they had not at their disposal the services of a 

 sympathetic and competent glass-maker. 



I have quite recently, by the courtesy of a friend, enjoyed the 

 privilege of reading a number of letters, I believe as yet unpublished, 

 written by Abbe, during the period of his work on optical glass, to a 

 well-known English microscopist, now dead. One of these letters, dated 

 October 9, 1S81, is very interesting because it sets out very clearly the 

 high-water mark in optical construction attained by optical glasses 



a 



Nature, March -30, 1916, pp. 100-1. 



