628 SUMMARY OF CURRENT RESEARCHES RELATING TO 



MICEOSCOPY. 



A. Instruments, Accessories, etc.* 

 il) Stands, 



Spencer Mon-objective Binocular Microscope, f — The great 

 feature of this instrument is that it is a binocular adapted for use with 

 any standard microscope objective, from the lowest to the highest power 

 oil-immersion objective. Both eyes are used at all times. The oculars 

 are separated by simply turning a knurled ring on the right-hand tube, 

 which is the most convenient position possible, involving a horizontal 

 sliding movement capable of accommodating any pupillary distance from 

 50 to 75 mm. A knurled ring on the left tube serves as a correction 

 collar to focus for the left eye independently of the right eye. A shutter 

 is fitted inside, just below each eye-piece, operated by little handles at 

 either side of the instrument. These shutters serve a double purpose — 

 to be absolutely sure that one is seeing equally well with each eye ; also, 

 by closing of the left eye, to focus for the right eye by means of this 

 side fine-adjustment. The tubes containing the eye-pieces are set at an 

 angle of 4° from the perpendicular, which brings the point of conver- 

 gence about 17 inches from the eye at the average pupillary distance. 

 The eyes work normally without strain, and instantly blend the two 

 images into one with the same ease as they do when viewing any ordinary 

 object. 



The instrument is made in two models, Nos. 1 and 2. No. 1 is 

 fitted with a revolving mechanical stage, 120 mm. in diameter ; and 

 No. 2 has a plain rectangular stage, 110 by 112 mm. 



(2) Eye-pieces and Objectives. 



Notes on the Calculation of "Thin" Objectives.^ — In this paper, 

 T. Smith arrives at formula for the calculation of the curves of thin 

 achromatic objectives, when two aberration conditions have to be satis- 

 fied. A two-lens objective satisfying the conditions as a rule cannot be 

 cemented, because the curvatures of the inner surfaces are unequal. He 

 gives formulge for the calculation of a triple objective satisfying the 

 conditions, but with only two glass-air surfaces, l^merical examples 

 are worked out, illustrating the application of the formulae to the calcu- 



* This subdivision contains (1) Stands ; (2) Eye-pieces and Objectives ; (3) 

 Illuminating and other Apparatus ; (4) Photomicrography ; (5) Microscopical 

 Optics and Manipulation ; (6) Miscellaneous. 



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



X National Phys. Lab., Collected Researches, xiii. (1916) pp. 181-94 ; and Proc. 

 Phys. Soc. Lond., xxvii. pt. 5 (1915). 



