PKOCEEDINGS OF THE SOCIETY. 267 



ocular, an eikonometer furnishing an image of the stage micrometer 

 focussed upon a measuring scale. 



Exhibit 2. — Arrangement for measuring the magnifying power of 

 the ocular. A Microscope with dismounted objective, with a measuring 

 rule placed athwart the open extremity of the barrel ; and another 

 measuring rule disposed in the plane of the Ramsden disk of the eye- 

 lens. 



Exhibit 3. — Arrangement for measuring the magnifying power of a 

 pocket-lens by the aid of the eikonometer. A Zeiss pocket-lens magni- 

 fying 6 diameters is disposed so as to give, at its full focal distance, an 

 image of a millimetre scale disposed upon the table. Arranged above 

 •the pocket-lens an eikonometer showing the. image of the millimetre 

 scale covering G (millimetre) divisions of the measuring scale. 



Exhibit 4. — Arrangement for measuring the magnifying power of a 

 lens by means of a median intercostal line developed by the aid of a 

 diffraction grating. 



A. A diffraction grating ruled 400 lines to the inch is taken before 

 the eye, and is held with its rulings parallel to the lines inscribed upon 

 ■a sheet of paper placed at 10 in. from the eye. The lines on the paper 

 being in each case ruled in pairs placed at different distances apart, 

 there is furnished by each of the lines (except in the case presently to 

 be considered) a series of six lines, viz. two principal or dioptric images 

 each flanked on either side by two diffraction images. In the case where 

 the object lines are placed at an appropriate distance apart, the centrally 

 disposed diffraction images are shown to coincide in position, giving 

 ■origin to a single conspicuous median intercostal line, instead of, as in 

 the case where the lines are placed too near together, or too 'far apart, 

 two separate and less conspicuous median lines. The distance between 

 the object lines which furnish the conspicuous single median intercostal 

 line is measured off by means of a millimetre rule. 



B. Everything is maintained in position except in the following 

 respects, (a) A system of more closely interspaced paired lines is sub- 

 stituted for that previously employed, {b) The lens whose magnifying 

 power is to be measured is taken in hand, and is so disposed as to yield 

 the largest possible erect image of the object lines. A search is then 

 instituted among these paired lines until a pair is found which yields 

 a single conspicuous median intercostal line. 



By the aid of a millimetre scale the interval between the pair of 

 lines which yields the intercostal line in question is now measured. 



The quotient obtained by dividing the linear distance measured in 

 A into the linear distance measured in B corresponds to the magnifying 

 power of the lens. In the case of the Zeiss pocket-lens ( x C diameters) 

 employed, the quotient is found to be 6. 



Exhibit 5. — Arrangement for measuring the magnifying power of 

 the Microscope by means of the median intercostal line. 



A Microscope is focussed upon an Abbe diffraction grating, the 

 wider interspaced lines of the central ruling appearing in the field of 

 "view. 



A diffraction grating ruled 400 lines to the inch is interposed 

 between the eye and the eye-lens of the Microscope, and the grating 

 is rotated until the diffraction images of the object lines fuse together 



