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Draw-tubes versus Deep Eye-pieces. 
tiling, — the difference being in terms , not in results. Both mean 
spontaneous cell formation. Both alike deny that a cell is necessaiy 
to the development of a new cell ; hut, while the former comes 
squarely to the point and says what it means, the lattei makes use 
of the molecule as a sort of sugar-coat for that unpalatable dose, 
spontaneity. — The Lens, July. 
VIII. — Draw-tubes versus Deep Eye-pieces. 
By M. A. Prazmowski. 
The magnifying power of any microscope depends upon the three 
following conditions : — 
1st. The focal distance of the objective (equal to the principal 
focus of a small equivalent lens placed in the optical centre of objec- 
tive combination). 
2nd. The magnifying power of the eye-piece. 
3rd. The distance between the optical centre of the objective 
combination and the eye-piece. 
By varying the last two conditions, for instance, a long tube and 
low eye-piece, or a short tube and a high eye-piece, we may obtain 
results identical, so far as magnifying power is concerned. In other 
words, designating by L and 1 the longer and the shorter tubes, 
and by 0 and o the stronger and feebler eye-pieces, q — ~ • 
For the sake of convenience, we leave out of this statement the 
slight difference in the power of the objective produced by the 
movements of the collar for the purpose of obtaining the best defi- 
nition with the two eye-pieces. 
It is a practical fact that, with a given magnifying power, the 
image obtained by a short tube and high eye-piece is more perfect, 
clearer, and brighter than that obtained by the use of a long tube 
and low eye-piece. 
Any observer may verify this fact by employing successively 
the two methods for the purpose of augmenting, or perhaps we may 
say exaggerating, the power of a given objective beyond its best 
working limits. It will be found that the richness of detail is better 
preserved with the short tube and high ocular than with the long 
tube and low ocular. 
By a careful study of the conditions present in the relations of 
the eye-piece and objective, I think I am able to show that the 
