114 CORRESPONDENCE. 



from infested territories of anything that might serve as a vehicle for 

 the insect. 



How to make Sections of the Potato showing Structure. — Mr. 



Taylor has given the following method in ' Science Gossip ' for July. 

 He has recently shown that the vascular bundles in a potato may of 

 course be easily seen by cutting a potato in two through its axis (the 

 section also passing through some of its "eyes"), and coating the cut 

 surface, first with a solution of bichromate of potash, and afterwards 

 several times with a strong tinctiu'c of iodine, which will stain the 

 starch blue, but leaves the vascular bundles yellow. The air-ducts 

 will then be seen to extend invariably to the eyes. For microscojiical 

 study these sections are to be made and treated with a strong acid or 

 caustic alkaline solution, which will dissolve the starch, but leave the 

 bundles unaltered. The sections may then be mounted as usual. To 

 isolate the vascular bundles, place a potato, skinned without wounding 

 the " eyes," in a solution of sugar and water (two ounces to the pint) 

 and keep it at a temperature of 72° F. for nearly a fortnight. The 

 fungus of fermentation will reduce the potato to a pulp, except the vas- 

 cular bundles, which may be mounted in gum or balsam, and studied 

 with a power of one hundred diameters. 



COEEESPONDENCE. 



Immersion Aperture on Objects in Balsam. 



To the Editor of the ^Monthly Microscopical Journal^ 



Sir, — Further discussion with Mr. Tolles on this question is 

 needless, because by his own work he has settled it to my satisfaction. 

 He has probably secured some believers in his triumph and demon- 

 strations, and to theii' congratulations I now leave him. So long 

 as imaginary diagrams are used evading the true form of an object- 

 glass, with wrong dimensions and focal distances, futile and endless 

 arguments may be advanced. Including Mr, Tolles' last illustra- 

 tions in this category, I do not care to be at the trouble of translating 

 his text to arrive at his probable meaning, though I perceive that 

 he is now struggling for a few degrees only. He long ago stated that 

 in practice he had actually secured the disputed extra theoretical im- 

 mersion apertures on balsam-mounted objects, with glasses of his own 

 construction. His recent production having been placed in my hands 

 by Mr. Crisp, I have shown by actual measurement, and by cutting 

 oif all false rays by a suitable stop, that such apertures had not been 

 produced, so that " the fact becomes clear and indisputable." 



It is very easy for Mr. Tolles to remark that this stoj) is " a mere 

 contrivance to express my sensations," and " that the whole seems to 

 him quite unnecessary." This I might have exiiected. But this stop 

 always is tlie focal plane of the object, and admitting all rays up to 180° 

 from that point, will serve as a salutary check against any optician 

 who may choose to vaunt his glasses on the questionable merits of 



