Remarhs on a " Note on Amfhipleiira peUucida," &c. 225 



eye lens ; and for registering the readings of the lines the direct 

 rays of the sun should be thrown up into the slit, made as narrow 

 as is compatible with sufficient light, and when the prisms are put 

 in place it is best to focus on the E line as sharply as possible, this 

 line being near the middle of the spectrum ; this is best done by so 

 focussing as to be able to see as many of the fine lines in the neigh- 

 bourhood of E as possible. Now by means of the reflector make 

 the bright line or spot of the micrometer visible, bring it over E, 

 just above the spectrum, focus it by its lens so that it is sharp and 

 quite free from flame, and so that a lateral to-and-fro motion of the 

 eye does not displace it by parallax ; this is most important, all ac- 

 curacy will depend upon it. Now see that all the fittings of the 

 instrument are firm and as free from spring as possible, and with a 

 light hand bring the spot over E, taking care that no part of the 

 instrument but the micrometer screw is touched, and do not let 

 the eyebrow touch the eye-piece, as this may shift the spot. Ha-sang 

 registered the best reading by many averages of E, that of A B C D 

 and F G should be each sejxirately and directlij determined from E, 

 and bringing the spot back again to E to test its reading. If 

 great care is taken in the first instance in determining the true 

 relative positions of these principal lines it will be well bestowed, 

 and any of the smaller lines can be registered by taking the nearest 

 principal Hne as a standard. 



It may be found necessary to adjust the focus of the micrometer 

 lens when the spot is over the extreme positions ; it must then be 

 started again from E as a point of departure. 



It will be found on using the instrument another time that the 

 spot will not give the exact registered reading of E ; it must be 

 made to do so by a shght side pressure or twist given to the eye- 

 piece cap carrying the micrometer. Hence the importance of having 

 the instrument in its firmest condition before finally determining the 

 reading of E. 



The accompanying Plate (CIV.) has been carried out on the 

 plan described above, and explains itself safficiently. 



VIII. — Some Remarks on a ''Note on the Resolution of Amfhi- 

 Ijleura i^eUucida hy a Tolles' Immersion ^th. By Assistant- 

 Surgeon J. J. Woodward, U. S. Army." By Edwin Bicknell. 



Theke are some points in the " Note " of Dr. Woodward under 

 the above title, which, I think, will bear a few remarks. I have 

 been much interested for the past three years in comparing and 

 measuring different objectives by diflerent makers, in order to 

 ascertain how near the " nominal power," as rated hj the maker, 



R 2 



