704 SUMMARY OF CURRENT RESEARCHES RELATING TO 



be clamped rigidly, and a slight lateral push on the feed-wheels will 

 rotate the lamp on its support. Small carbons burn away more rapidly 

 than large ones, and the feed requires more frequent attention. The 

 carbons shown in the engraving are T 7 ^ and T \ in. in diameter, " Niirn- 

 berg soft-cored Electra brand." In the highest power work which the 

 author has done with the electric light, a very steady light was obtained 

 by using a -^ in. cored vertical and a -^ in. solid horizontal carbon. 

 The light from a 110-volt alternating incandescent current was sufficient 

 to give a strong picture of a stained transverse section of an earthworm 

 having a magnification of 8800 diamefers by measurement. The objec- 

 tive used was a B and C xV"^ n - oil-immersion in connection with an 

 amplifier. The conditions just described will indicate the reason for 

 the use of hand-fed 90° arc lamp rather than for other types of hand- 

 fed lamps or for any of the automatic-fed. The automatic-fed lamp 

 is convenient in very low power and lantern slide projection, but here 

 also the 90° lamp gives as good results. Concerning the field of illu- 

 mination on the screen, it should be noted that, if the Microscope is 

 pushed too close to the lamp, the field will be blue and will not give 

 satisfactory results. As the Microscope is moved to a greater distance 

 from the lamp, the centre of the field will be strongly illuminated, and 

 at a still greater distance the entire field will be evenly illuminated, 

 and this is the best position for all objects except ths most difficult, 

 which may require the strong central illumination. The system of con- 

 densers is of the utmost importance, and the best combination consists 

 of three plano-convex condensers, each 4^ in. in diameter, and arranged 

 as follows : — No. 1, a plano-convex lens of about 5 in. focal length 

 with its plane side next to the light ; No. 2, a plano-convex lens of 

 6 J, in. focal length with its convex side facing the convex side of No. 1, 

 and enclosed in the same cell with it ; No. 3, a plano-convex lens of about 

 11 in. focal length and attached to the water-tank with its convex side 

 facing the Microscope. A simple plano-convex sulstage condenser of 

 x£ in - focal length is a necessity in high-power work with the above 

 condensers, but its efficiency varies with its distance from the object. 

 The correct distance for each objective should be determined by ex- 

 periment and recorded for reference. All the above directions for the 

 electric light apply equally well to the oxyhydrogen, in which the 

 luminous point is constant : but the less interne light materially reduces 

 the available magnification. 



Method of Measuring Objects in the Microscope.* — F. E. Ives 

 proposes a simple arrangement for stage measurement. In fig. 134, 

 A represents the foot of the Microscope ; B is a block of wood notched 

 to fit against the foot and project in a particular direction ; C is a rider 

 with set-screw and a post, and spring-clamp D to hold a jeweller's saw 

 having sixty-four teeth to the inch ; E is the Welsbach light diffused 

 by a ground- glass chimney and shielded from the eyes by a hood open 

 only on the side towards the Microscope. The source of light being 

 on a level with the Microscope, the jeweller's saw is supported in a 

 vertical plane directly between the light and the Microscope mirror, 

 and in this position its image can be focussed in the field of the Micro- 



* Journ. Franklin Inst., cliv. (1902) pp. 73-6 (3 figs.). 



