ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 629 



lation of an astronomical object-glass, and the kinds of glass are deter- 

 mined with which a cemented doublet satisfies the conditions for this 

 case. 



Spencer Demonstration Ocular."' — This apparatus, as its name 

 implies, is for demonstration purposes. It is a short tube, with an eye- 

 piece at one end and a cap at the other. The cap is attached to the top 

 of the body-tube of any compound microscope. The instructor looks 

 through the eye-piece, directly over the tube, and at the same time the 

 student looks through the eye-piece at the outer end of the demonstration 

 ocular. A pointer, conveniently operated by a small knurled handle just 

 below the eye-piece (over the microscope tube) is operated by the 

 instructor, who is thus able to point to the exact detail to which he 

 desires to call attention. 



Choice of Glass for Cemented Objectives. f — It is well known that 

 a telescope objective consisting of two compound lenses can be so con- 

 structed as to be simultaneously free from chromatic and spherical 

 aberrations and from coma, provided proper forms are given to the 

 components. When these are to be cemented together the three con- 

 ditions can only be satisfied if a careful choice is made of the kinds of 

 glass used. Tables have been published giving all the particulars 

 necessary for the construction of such objectives when the corrections 

 are required for an object at infinity. T. Smith's note, under the above 

 title, deals with the changes in the kinds of glass to be employed when 

 the objective is required to be free from these aberrations for objects at 

 some finite distance. 



(S) Illuminating' and other Apparatus. 



Spencer Delineascopes.| — The Spencer Company have lately added 

 Models 2 and 3 to their previous series of Delineascopes, or projection 

 lanterns. No. 3 is a larger model than No. 2. The following notes 

 refer to No. 2. This apparatus accomplishes both "opaque " and lantern- 

 slide projection. The illuminated "opaque " area is 6 x 6 inches and 

 will accommodate postcards, photographs, books, maps, small objects, etc. 

 By an entirely new construction of base the entire operating end and 

 sides of the apparatus are open and free from obstruction, thus facilitat- 

 ing ease of handhng. The illumination is obtained from a Mazda bulb 

 in its Mogul porcelain receptacle, and by an ingenious arrangement of 

 interior mirrors the illumination of the. opaque material and its screen 

 picture is 20 to 25 p.c. more brilliant than is usually obtained with 

 ■similar apparatus. The Mazda is gas-filled and of 400-volt power, and 

 is attachable to any 110-volt incandescent lighting socket. Alternating 

 or direct current may be used. The large projection objective is 4 inches 

 in diameter and 16 inches equivalent focus. The method of supporting 

 and focusing this large objective is entirely new, and is said to be free 

 from the usual objections and to give results which are all that can be 



* Spencer Lens Co., Buffalo, N.Y., Catalogue, 1917. 



t National Phys. Lab., Collected Researches, xiii. (1916) pp. 197-208 ; and Proc. 

 Phys. Soc. Lond., xxviii. pt. 4 (1916). 



X Spencer Lens Co., Buffalo, N.Y., Catalogue, 1917. 



