318 



MR. J. S. BUDGETT ON THE STRUCTUKE 



figures of Plate XXXIII., and figs. 9 and 10 of Plate XXXIV., were drawn from 

 reconstructions by Kerr's method ; that is, the sections were drawn on sheets of ground- 

 glass by the aid of the camera lucida and a medium, of the same refractive index 

 as the glass, run in between the sheets of glass i. 



I have found it extremely convenient to use this method in the following way : — 

 Five dozen sheets of ground-glass 4 inches square and -^^ inch thick are ground 

 and polished en masse, so that their cut edges when placed together form plain 

 surfaces. 



After having draAvn the first section, the image of the second section is exactly 

 superimposed on the drawing of the first, which is then removed, and the next glass 

 plate being put exactly in its place and the drawing having been made the image of 



Text-fig. 2. 



Apparatus 1186(1 in reconstructing. 



the third section is exactly superimposed on the drawing of the second, and so on. 

 In this way all the successive sections have the same register as the first, and when 

 placed together in the form of a cube the magnified image of the object is formed. 



The cube of glass plates is then transferred plate by plate to a cubical glass jar, the 

 width of which is a little more than the diagonal of the plates, so that the cube of 

 glass can just freely rotate within the square vessel. The vessel contains a mixture 

 of fennel-oil and oil of cedar, which I have found to be sufficiently near the refractive 

 index of the glass, and to have the advantage over clove-oil that it does not become 

 dark with age. 



The cube of plates is held in position by means of the glass turn-table shown in 

 text-fig. 2. It consists of a circular disc of plate-glass, the diameter of which is very 



' Quart. Journ. Micr. Science, vol. slv. pt. 1, p. 6. 



i 



