CHAP. IV.] MICROSCOPIC PREPARATIONS. 35 



balsam was then placed upon it; the insect, whilst yet alive, 

 was then placed on the balsam, and the glass was again very 

 gently warmed in order to kill the insect ; another piece of the glass 

 was then heated over a candle and placed on the insect, when the 

 creature was hermetically sealed up for ever. 



It is necessary (lie says) that the insect should be dry when it is 

 mounted, and we must take especial care not to apply too much heat, 

 which will corrugate the antennae and destroy the form of the insect. I 

 strongly recommend to all entomologists this mode of preserving small 

 insects; and having once properly secured them, they will last for an 

 indefinite period, and can be handled without the slightest risk of 

 injury.* 



Mr. Smee mounted many hundreds of these microscopic 

 preparations of the Aphis vastator, and of slices of diseased 

 potatoes ; and these slides have, after the lapse of nearly thirty 

 years, been the means by which this great controversy on the 

 potato disease has probably at last been settled. For in the winter 

 of 1876, when Mr. Worthington Smith was investigating the 

 subject of diseased potatoes, my father placed in his hands for 

 examination 360 slides of diseased potatoes and of aphides, all of 

 which the latter had himself mounted during the great potato 

 murrain of 1846-1847. On placing these slides under a 

 powerful microscope, Mr. Smith discovered that some of the 

 aphides were completely filled with the fungus internally and' 

 covered with it externally, and that gentleman has further demon- 

 strated that this insect punctures the potato, and inserts in it the 

 fungus. A full account of these recent observations of Mr. Smith, 

 together with two drawings which that gentleman has kindly 

 made for me from my father's mounted specimens of the Aphis 

 vastator, and of a diseased potato showing the resting spore of 

 the fungus within the aphis, will be found in the Appendix, 

 No. XY.c. By this it would appear that the primary cause of 

 the potato murrain of 1846-1847 was the aphis, and the 

 secondary cause the fungus. 



The following question, which my father addressed to a well- 

 known actuary, is transcribed for the amusement of those who 

 may be fond of figures : 



An aphis arrived on my cucumber, January 1, 1861. It had ten young 

 ones at the end of ten days, ten more in ten days' time, and every suc- 

 ceeding ten days. Every young one had ten young ten days after birth, 



* See ' Potato Plant,' p. 14. 



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