850 SUMMARY OF CURRENT RESEARCHES RELATING TO 



be readily turned away when not required, and a tube inserted in 

 its place for sub-stage apparatus. The fitting of the polarizer is 

 graduated and a spring catch indicates when the prisms are 

 crossed. The rotating glass stage is graduated, and has a " self- 

 centering " arrangement. Two sliding boxes at the lower end of the 

 body-tube serve to carry the analyzer and, below it, a Klein's quartz 

 plate, which can thus be readily slipped in and out. An extra 

 analyzing prism with divided circle is placed over the eye-piece 

 (which has crossed spider-lines) with a contrivance for rotating 

 crystals between it and the prism. 



There is also a new arrangement for showing the rings in biaxial 

 crystals of extreme wide angle, diopside for instance, with its entire 

 system of rings being (it is claimed) " exhibited as large and with 

 greater brilliancy than with Norremberg's Polariscope." For this 

 purpose an achromatic lens is interposed by means of a supplementary 

 draw-tube between the eye-piece and objective, an optical combination 

 of large aperture being fixed over the polarizer. 



Mr. Bulloch, of Chicago, has also issued a Microscope for the 

 study of rock sections, adapting for the purpose the model shown in 

 Fig. 140 of Vol. III. (1880) p. 1077. Cf. also Eutley's, Vol. II. 

 (1879) p. 470, Nachet's Petrographical Microscope, Vol. III. (1880) 

 p. 227, and Verick's Goniometrical Microscope for Mineralogy, 

 Vol. I. (1881) p. 812. 



" Jumbo" Microscope. — The instrument, from Mr. Crisp's col- 

 lection, shown in Fig. 156 (about ^ nat. size) is another of the numerous 

 instances of misdirected ingenuity in the designing of Microscopes. 

 It was made in 1851 by G. Lowden, junr., a Dundee optician, for a 

 gentleman then lately returned from India. It stands 4 feet high, 

 weighs l£ cwt., and the body-tube is 4 inches in diameter. It is 

 therefore entitled to the distinction of being the largest and heaviest 

 Microscope made within modern times ! * 



For the coarse adjustment the stage is moved up and down along 

 the bar which supports the body-tube, its movement being controlled 

 by the large milled head on the upper end of the bar. The fine 

 adjustment is worked by the milled head and rod attached to the body- 

 tube, by which an inner tube carrying the objective is raised or 

 lowered. 



As the stage is so far from the observer its movements are effected 

 by the two longer rods terminating in milled heads shown in the 

 figure above the end of the bar. One of these moves the stage from 

 back to front, the other turning it to either side on a pivot at its base, 

 giving it therefore a movement in a segment of a circle. The re- 

 maining shorter rod has a screw at its lower end which, working in 

 a toothed wheel on the axis, causes the body of the instrument to 

 incline as may be desired. The eye-piece is pierced with a slit to 

 admit a slide holding prepared paper for "calotyping" an object by 

 the old paper process. 



* Schott, 'Magia Universalis,' 1677, describes and figures Microscopes of 

 enormous size. 



