396 Notes. 



On Nelson's New Formula Amplifier. 

 By A. A. C. Eliot Merlin. 



A few months ago, when endeavouring to effect some delicate 

 microscopical measurements, it was found that the magnification 

 afforded by even y 1 ^ and y 1 ^ in. objectives used in conjunction with 

 the low-power eye-piece of Powell's screw micrometer, was in- 

 sufficient always to insure the greatest possible accuracy. It is- 

 obvious that for such work a very high magnification is advan- 

 tageous, as then an entire revolution of the divided drum, and 

 consequent considerable movement of the spider line, represents a 

 very minute real interval, and therefore small errors in bringing 

 the "wires" into exact contact with the margins of the object to 

 be measured are reduced to a minimum. In ordinary cases, for 

 visual purposes, it is an easy matter to obtain any useful amount 

 of enlargement that may be required with a given objective by the 

 simple expedient of employing an ocular of suitable power ; but 

 with the screw micrometer any great augmentation of power at 

 the eye end of the instrument must inevitably cause the spider- 

 lines themselves to appear coarse and thick, a consequence which 

 certainly would not tend to increase the accuracy of results obtained 

 under such conditions. 



In the circumstances I appealed to my friend, Mr. E. M. Nelson, 

 asking him whether it would not be feasible to employ some suit- 

 able kind of amplifier over the back combination of the objective, 

 thus insuring a sufficient increase of power with various lenses 

 without affecting the apparent fineness of the micrometer spider- 

 lines. In response, Mr. Nelson most kindly computed for me a 

 simple form of amplifying lens, which has proved so satisfactory 

 in practice that I venture to draw the attention of Fellows of the 

 Society to its capabilities. 



The lens is negative, plano-concave, and in use is placed in my 

 instrument (Powell's No. 1) on the top of the fine-adjustment 

 tube, at a distance of about three inches above the back combina- 

 tion of the objective, and in that position increases its initial 

 magnifying power 2 ■ 52 times. The effect of the amplifier is also 

 to increase the working distance of the objective and thus disturbs 

 its corrections for critical work ; but this disturbance can be com- 

 pensated by means of the correction collar, and therefore adjustable 

 objectives are preferable for use with the amplifier in order to 

 obtain the best possible definition. When, however, the objective 

 is in a fixed mount, the necessary correction may be effected by 

 lengthening the tube as for decrease in thickness of cover-glass. 



