CORRESPONDENCE. 35 



a few drops of gold size may be added with advantage. I would, 

 advise the use of zinc white, in preference to white-lead, as the latter 

 turns yellow in the course of a month or two. I give the preference 

 to vermilion or purjile lake for the exterior ring, finishing with an 

 interior ring of zinc white. 



My experience does not confirm that of Dr. Hallifax ; I always 

 find sealing-wax varnish to become brittle in the course of a year or 

 two. 



The following cement is given in ' The Microscope' by Dr. Frey. 

 Thiersch's cement. — Dissolve shell lac in spirit of wine, in sufficient 

 quantity to make a thick varnish, colour with a concentrated solution 

 of aniline blue or gamboge, in absolute alcohol ; add about a scruple 

 of castor-oil 'to each ounce of mixture. After some further evapora- 

 tion it must be preserved in a well-closed vessel. Previous to using 

 this cement the inventor directs that the edges of balsam-moimted 

 slides should liave a layer of balsam dissolved in chloroform, jmt 

 round them in the same manner as asphalte, and at least three days, 

 but still better, weeks or months should be allowed to elapse before 

 applying the cement. 



Yours very truly, 



F. KlTTON. 



Potato Blight, 



To the Editor of the ^Monthly Microscopical Journal.'' 



London, Dec. 12, 1873. 



SiB, — It is remarkable how little has been done with the micro- 

 scope for the investigation of the origin and spread of the potato 

 blight. This, like many others of a similar character, has been rather 

 hastily attributed to the growth of a fungus— I consider without suffi- 

 cient reason. 



A fungus, from the universal presence of the spores in damp 

 localities, and its rapid growth, may ajjpear simultaneously with 

 morbid conditions, and yet not be the primary cause. The vine dis- 

 ease, being cuticular, may be readily traced by the microscope to a 

 fungoid origin, further proved by the well-known sulphur cure, so 

 destructive to fungi in confined localities. This is of no avail in the 

 potato disease, which, under conditions favourable to its development, 

 is internal and constitutional. 



On placing a very thin slice of potato (taken at any time of the 

 year) under the microscope, the cells are seen filled with starch 

 granules, and the walls coated with a layer of active protoplasm of 

 the usual molecular appearance. In the healthy cell, this protoplasm 

 (the vital principle in all plants) when seen under the highest powers, 

 with suitable illumination, has a vibratory motion, with feeble currents, 

 in various directions. On approaching the vicinity of the diseased 

 portion, the cell walls begin to appear of a light brown colour, and 

 wherever the least tinge of this becomes apparent, there is no movement, 

 nor can any protoplasm be detected adhering to the wall of the cell, 



