68 PLANT PHYSIOLOGY 



for manipulation of the slide; it was warmed by heating water between the 

 walls from a lamp beneath, and cooled by ice and salt between the walls. 

 Much improved in details by various workers, it may now be bought 

 in several forms, under the name Microscope Thermostat, made by Rohr- 

 beck of Berlin. It is, however, not readily cooled, and is better adapted for 

 keeping constant temperatures than for changing them. Very simple was 

 the arrangement used by Velten (1876), consisting of a crystallizing dish 

 of water, which can be cooled or warmed by flowing water, and in which, 

 wholly immersed, was the object, the objective, and part of the thermometer 

 tube; but it required the use of the now rare water-immersion lens. Pfeefer 

 (Zeitschrift fur wissenschaftliche Mikroskopie, 7, 1890, 433) has used it 

 with improved details. Theoretically an efficient form could be constructed 

 on the principle that object and thermometer bulb should lie side by side, 

 embedded in suitable openings in the same glass plate and surrounded by one 

 continuous film of water, but such apparatus is very difficult to manufacture. 

 Of normal forms by far the best is that made by Verick (figured by 

 Goodale, 202), a double-walled box surrounding a place for the slide and 

 heated by circulation of water from a reservoir; thermometer and object 

 being in separate compartments, their temperatures need not be the same, 

 though in fact they agree so nearly as to make this stage in practice a pre- 

 cision instrument. More or less modified, it has been used by Ewart (60) 

 and others, and is sold by various makers in a variety of forms, most of them 

 taking the slide not in a chamber, but upon the surface, e.g., the Stricker 

 form. Far less accurate is the Schultze form (1865, figured in Verworn, 

 392), consisting of a flat metal plate with a thermometer in contact; it permits 

 a considerable difference in temperature between object and thermometer, 

 and cannot readily be cooled. I have myself devised a form which is amply 

 accurate and convenient for student use (Botanical Gazette, 27, 1899, 257); 

 somewhat improved, it is among my normal apparatus (page 46) and is 

 figured herewith (Fig. n). It is made from a single sheet of copper, 

 brought to the form shown by the figure; the prepared slide is placed in the 

 flat chamber, where it can be moved about by the right thumb until the 

 object is in position, when it is lighted from below through openings left for 

 the purpose. The thermometer is placed with its bulb near the middle of 

 the cylindrical chamber, and should sag a little so the bulb may press against 

 the metal. To heat the stage, water is placed in the box and warmed by a 

 spirit-lamp slowly moved in from the tip, and to cool it, broken ice, and later 

 salt, are slowly added in place of the water. The stage is firmly clamped to 

 the microscope stage with a non-conducting felt mat intervening. To reduce 

 the temperature of the object below zero, it is necessary to place the micro- 

 scope in a cool place, or, better, to enclose the stage in a tight-fitting, non- 

 conducting, cloth cover. If properly used, the temperature variation between 

 thermometer and object should not exceed i° to 2° and this may be reduced 

 in using low powers by covering the chamber with a cover-glass, the prox- 

 imity of the radiating or absorbing objective being the greatest cause of 

 temperature error. Other precautions already mentioned above should be 

 observed in its use. 



