270 



SUMMAEY OP CUERENT RESEARCHES RELATING TO 



service in many other branches of thermal microscopy where the exact 

 determination of the temperature applied is desirable, e. g. as sug- 

 gested by Mr. A. H. Elliott, in the determination of the melting point 

 of rare chemical substances, &c. For this purpose, the apparatus in 

 Fig. 51 might be supplied with another tube, on the opposite side to 

 those represented, through which might be inserted, beneath the 

 objective, a small glass tube containing the substance to be examined, 

 and thus immersed, by the side of the thermometer bulb, in the water, 

 oil, paraffin, or other liquid which the circumstances may require for 

 the bath." 



Beck's Achromatic Condenser for Dry and Immersion Objectives. 



— In an earlier form of (dry) condenser (Fig. 53), Mr. Beck made 

 use of a revolving front rotating a series of lenses mounted on a plane 



Fig. 58. 



Fig. .54. 



surface over the back combination. This plan was, however, only 

 available for a dry condenser ; if used for immersion, the connecting 

 fluid would be drawn away by capillary attraction. 



To avoid this inconvenience, the new form shown in Fig. 54 has 

 been devised, the movable series of front lenses being mounted 

 in a segment of a sphere and rotated by a milled head acting on a 

 pinion and toothed disk. The first lens, when brought over the back 

 combination, has a low angle, and is intended for use without fluid for 

 histological objects. By revolving the diaphragm, the angle can be 

 varied from 35° to 7°. The next is a full aperture lens with which, 

 by revolving the diaphragm, the angle can be varied from 180° down- 

 wards. The third lens, with full aperture of diaphragm, has an 

 angle of 110° in glass = 1*25 N.A., and is truncated, cutting out the 

 central rays. The fourth lens has also an aperture of 1 * 25, and is 

 truncated, so as to stop out all rays up to 180° in air. The fifth is 

 similar to No. 3, but the periphery is painted over, so aa to allow 

 pencils only at right angles to pass. 



Pennock's Oblique Diaphragm.* — Mr. E. Pennock suggests an 

 adaptation of Mr. Mayall's spiral diaphragm,t to be attached to the 



* Amer. Journ. Micr., vii. (1881) p. 161 (3 figs.). 

 t See thia Journal, i. (1881) p. 126. 



