256 



FRANCIS DARWIN. 



* Warm-kasten '^ is described in Sachs' ' Text-book of Botany/ 

 Eng. Tr. p. 658. Velten's instrument is fully described and 

 figured in ' Flora' (loc. cit.) 



In my observations on the filaments of the teasel I have 

 made use of both of the above apparatus ; in this way, no 

 serious error can occur. If the filaments were liable to contract 

 spontaneously it would, of course, be impossible to make any 

 trustworthy experiments, but this is distinctly not the case ; the 

 filaments remain extended with great constancy for long periods 

 of time. Again, if they contracted when washed to-and-fro by 



^ Mr. Hawksley has constructed a box of this description for me, which 

 answers admirably. At his suggestion it was made of carefully soldered 

 tin-plate, which is said to be more durable than zinc if care is taken to 

 drain the water out thoroughly after use. Mr. Hawksley has also substi- 

 tuted tin-plate lined with felt for the cardboard cover. According to 

 Velten, the projecting portion of the tube of the microscope ought to be 

 enveloped in felt to prevent undue cooling of the objective. Velten's appa- 

 ratus is more simple than Sachs', and can be fitted up by any one for 

 himself. A small glass vessel is placed on the stage of the microscope, the 

 object to be examined is supported on a wooden frame, so that when the 

 vessel is filled with water the object is surrounded by it on all sides, and the 

 objective has to dip beneath the surface ofthe water like an immersion lens. The 

 temperature is regulated by a current of hot water flowing in at the bottom, 

 and a siphon carrying off the surface-water at the opposite edge of the 

 vessel. Both tubes are provided with taps, and the water should be made 

 to flow by drops and not in a quick continuous current. A thermometer is 

 kept in the water close to the object. Velten cemented his thin cover- 

 glass to prevent the preparation under observatioa being disturbed. 1 have 

 found it more convenient to employ the following device : — A piece of stout 

 lead-foil is cut into the shape shown in the woodcut. The object is 



Lead-foil clamp for holding a viicroscopic object between two thin glass 



covers. 



mounted between two thin glass covers and placed over the square aperture 

 in the lead-foil ; the four flaps are then gently bent up (the hinge-lines are 

 dotted on the figure) so as to clamp the glass slides together, and the object can 

 now be placed in water without any danger of the cover-glasses separating. 

 This plan has the advantage of leaving the preparation accessible to re- 

 agents after the heating experiment has been concluded, and from being 

 between two thin glasses the object is rapidly affected by the changes in tem- 

 perature of the surrounding water. 



