856 SUMMARY OP CURRENT RESEARCHES RELATING TO 



e. Lower the object till it nearly touches the slide, and by means 

 of the mechanical motions of the substage place it exactly where it is 

 wanted, when a slight touch at the lever-end of the bristle-holder will 

 deposit it permanently. 



d. Eeturn the point of the bristle-holder to the first slide, and re- 

 commence the above operations as long as may be desired. 



Objects may be searched for and selected under a low power, such 

 as a 1 in., a 1/2 in., or a 2/3 in., and if very small may be deposited 

 under the other tube under a 4/10 in., a 1/4 in,, or a 1/5 in. Those 

 who cannot use their eyes alternately, may shift one eye from one tube 

 to the other with insignificant loss of time. 



The principal advantages of the instrument consist in the rapidity 

 with which it becomes possible to pick up and put down small objects, 

 and in the great precision of the manipulations. By employing 

 duplicate slides of a same material, one being placed under each of 

 the tubes, it becomes easy to use the Microscope for comparative 

 observations in polariscopy and spectroscopy by adapting the micro- 

 polariscope or the micro-spectroscope to one tube alone, leaving the 

 other to be used as an ordinary monocular Microscope. Many com- 

 parative and biological researches may also be conducted under the 

 Microscope without the need of the frequent changes of lenses and 

 shifting of the slides so irksome in many cases to the working 

 naturalist. 



For the dissection of minute animals or plants, for histological 

 researches in general, in the hunt for nematodes or other minute forms 

 of life, for the picking-up of desmids, diatoms, protophytes, &c., and 

 for the grouping of these objects easily, rapidly, safely, and elegantly, 

 I believe that the twin Microscope remains as yet unrivalled. 



I sincerely hope that others may derive as much satisfaction from 

 the use of the instrument as I have myself, and that it may lead to 

 increased results both in useful and in beautiful work." 



Klein's Mineralogical and Petrological Microscopes.* — Prof. 

 C. Klein in the instrument shown in fig. 181, has combined all 

 the most valuable of the recent suggestions for this class of Micro- 

 scope. 



The body-tube has the arrangement of M. Bertrand's stand j for 

 introducing above the objective a quartz plate, a quarter undulation 

 plate, a Nicol prism, Bertrand lens, &c. The objective can be centered 

 by two screws at the nose-piece. The stand can be inclined, and has 

 both coarse and fine adjustments, the latter reading to 1/500 mm. 

 The graduated stage can be moved in rectangular directions, and the 

 amount of movement read to 1/iOO mm. It can be rotated by rack- 

 work or by the hand. The polarizer fits in a tube beneath the stage, 

 and can be adjusted by rack and pinion. 



Two smaller forms are shown in figs. 182 and 183. 



* Nachr. K. Gesell. Wiss. Gottingen, 1884, pp. 436-43. The Microscopes 

 are made by Messrs. Voigt and Hocbgesang, of Gottingeu. 

 t See this Journal, iii. (1883) p. 413. 



