120 SUMMARY OF CURRENT RESEARCHES RELATING TO 



any object in the centre with the ^-^ and finder, then replace the 

 inch objective, read the finder, say Etc triangle, then on placing any 

 other object with the inch objective on 'Ex, on putting on the -^^ it 

 will appear in the middle of the field. 



3. Again, if an object be seen in the finder on Bz with the inch, 

 the object must be transferred to E^ to transfer it from the ^ to the 

 yi^, and so on for a ^^. Say the inch finds it on Fr when it was 

 central in ^V. Then if any other object be placed on ¥r by the inch, 

 it will be seen central by ^V, and Ex for -^-^ will be Fr for ^^. 



Use as a General Finder. — If the clip on the stage slides down and 

 closes up so as to occupy the place where the object would be, a small 

 piece of platinum wire is to be let in and riveted ; then a small hole 

 as fine as a watchmaker's pivot is to be drilled or punched through 

 it. The position of this punctum should be a mean position, so as to 

 correspond with the centre of stage motion. If this cannot be done, 

 a small piece of brass is to fold over instead, carrying the platinum 

 hole. 



To use the Transfer Eye-piece. — Place an object with the inch on 

 the centre of field ; note the triangle where the hole is seen on 

 being folded over into the field of view, say it is (At). Then when 

 the hole is again placed at At the object will be in the centre of field, 

 and if it be put on Bz ~l will find it, and if it be put on Ex ^-^ will 

 find it, and if it be put on Fr the ^V will find it. 



Angular Aperture— a Correction. — In a recent number of the 

 ' American Journal of Microscopy,' Mr. Bragdon alluded to a ^-^ oil- 

 immersion, by Tolles, stating its aperture to be 148° in a medium of 

 index 1"525, which would very nearly = 1*5 " numerical." Mr. Brag- 

 don has since informed us that " 148° " was a misprint for " 141°," so 

 that the ^jj in question has a numerical aperture of about 1"43. 



Low Powers of Large Aperture. — We learn that Mr. Tolles, of 

 Boston, has recently completed a f objective of 0*58 numerical aper- 

 ture ( = 70° " angular " aperture), and that even with this large aper- 

 ture the field is fairly " flat." It is said to resolve P. angulatum with 

 oblique light direct from the lamp (that is, without any form of con- 

 denser) ; whilst, with a concave mirror used in the axis, it exhibits the 

 lines on ordinary specimens, giving resolution in all parts of the field. 



Notwithstanding its aperture, it has been constructed for use on 

 the usual Microscopes having the " Society " screw. We believe that 

 all previous attempts made in America to secure so large an aperture 

 with this focus have required a larger gauge of screw, so as to permit 

 the utilization of a back lens larger than the gauge of the " Society " 



Gundlach's Homogeneous-immersion Objectives. — Mr. Ernst 



Gundlach has recently forwarded to Mr. Curties a new ^, constructed 

 on a formula by which he states he will be able to secure longer working 

 distance and an angle of aperture approximating to 180° in a medium 

 of index 1 • 52 (that is to say, a numerical aperture approaching 1 • 52). 

 He claims for the | an angle of 140° ( = 1 "43 " numerical " nearly). 



