Your correspondent Cadéel is taken to task by Deutsch2 for a slight misquotation in his letter. Deutsch comFour frame enlargements from video imaging of brownian movement. The test specimen is suspended globules in milk, and the scale bar represents 10 um. Frame separation 0.5 seconds, and the high-power lens from Brown’s microscope is calibrated at a magnification of x17O. lt resolves particulates of 1.3 um diameter. The microscope is in the collection of the Linnean Society. plains that he is wrongly cited as having said that Brown’s particles were “too large . . _ e.g. pollen”, and that he actually said they were “too light". Deutsch has published his belief that Brown claimed to observe the phenomenon in the movement of pollen grains. and this common misconception is incorrect.
As Brown makes perfectly plain. his observations were of the intracellular granules within the pollen cells, and not of the entire grains themselves3. Furthermore, Brown took great pains to avoid external perturbations and was clearly aware of the problems that might be caused by currents induced by such factors. Each of Deutsch’s objections can be faulted by a careful consultation of Brown’s privately printed account".
The authority now cited by Deutsch5 produced an interesting -account, although it perpetrated the widespread belief that the simple microscope was not capable of generating images of sufficient resolution. Perrin writes that the phenomenon was described “very shortly after the discovery of the achromatic objective”. This comment is misleading.
As Brown makes clear, his observations were made -using the simple (single lens) microscope. He_had brief recourse to an early achromatic compound system. but soon returned to the single lens3. The “pseudo-brownian movement” postulated by Deutsch is certainly not recognized by Perrin, or by other major workers in the field. I am aware of no evidence that it exists.
Brlan J. Ford
Rothay House, Mayfield Road,
Eastrea, Cambridge PE 7 2A)C UK.
1. Cadée. G. C. Nature. 354. 180 (1991).
2. Deutsch. D. H. Nature, 357. 354 (1992).
3. Brown, R. A brief account of microscopical observations made in the months of June. July and August 1827 on the particles contained in the pollen of plants and Additional remakrs on active molecules (London, 1828).
4. Ford, J. The Microscope 39 (3 & 4), 161-173 (1991).
5. Perrln. J. Brownian Movement and Molecular Reality (London. 1910).
No comments:
Post a Comment