1 885.] 



MICROSCOPICAL JOURNAL. 



193 



stant interchange takes place between 

 the interior mass and the external 

 layer, and an equable distribution 

 of nutritive material is secured in 

 the bodies of the most rudimentary 

 and testaceous types. When it is 

 borne in mind that in none of the 

 families of rhizopods is the circula- 

 tion uninterrupted, and that it not 

 only continually varies in rate, but 

 frequently ceases altogether for a 

 time, it will, I think, be allowed that 

 any analogy between the phenomena 

 and a special circulatory force is al- 

 together discountenanced.' ( Further 

 ObserA^ations on the Distinctive Char- 

 acters, Habits, and Reproductive 

 Phenomena of the Amoiban Rhizo- 

 pods.' Bv the same author. Annals 

 and Mag. Nat. Hist.. Dec, 1863.) 

 Lastly, the results already referred 

 to ' could not take place if the two 

 phenomena, namely, the vital contrac- 

 tility of the proloplasm itself and the 

 circulating force by means of which 

 the granules are impelled, acted inde- 

 pendently one of the other. Did they 

 act independently, any cessation or 

 alteration in the one would not ne- 

 cessarily involve a cessation or altei'a- 

 tion in the other, but the circulation 

 of the granules would continue un- 

 checked even when the protoplasmic 

 mass had attained a state of perfect 

 rest. And, notably, when the direc- 

 tion in which the protoplasmic mass 

 had for a time been moving became 

 suddenly reversed, the direction of 

 the granular movement would remain 

 unaltered, at least for a period, were 

 the force producing it an independent 

 one. But the direction which the 

 granules continue to take under these 

 circumstances almost immediately 

 becomes reversed likewise, proving 

 thereby that it simply follows the di- 

 rection which has been imparted to 

 it by the protoplasm. It only remains 

 to be stated that these results are ob- 

 servable whenever a fresh pseudopo- 

 dium is projected ; every modification 

 in the direction taken by the current 

 of granules being distinctly referable 

 to some corresponding change in the 



form assumed by the protoplasmic 

 body generally.' (On the Rhizopods 

 as embodying the Primordial Type 

 of Animal Life. By the same author. 

 Monthly Microscopical Journal., 

 April, 1869.) 



London, August 12th, 1885. 



The Actinic and Visual Focus in 



Photo-Micrography with High 



Powers.* 



BY JACOB D. COX, LI.. D., F. R. M. S. 



We find it commonly said that 

 whilst the diflerence between the vis- 

 ual and the actinic focus is consider- 

 able when making photo-micrographs 

 with low powers, it is not appreciable 

 when using high powers. My expe- 

 rience does not accord with this 

 statement, and some notes upon my 

 own experiments may have interest 

 to others. 



If the statement had been that a 

 sharp picture may be taken when the 

 object is exactly in focus with a high 

 power, I should not take exception to 

 it, and I incline to think that this is 

 what has been meant. But a sharp 

 picture may be either a positive or a 

 negative of the visual image seen in 

 the microscope, and in my own work 

 so many examples have turned out to 

 be positives when I expected them to 

 be negatives, that I have been led to 

 make an investigation of the subject, 

 in which the evidence tends strongly 

 to show that with our best high pow- 

 er lenses the image fixed upon the 

 sensitized plate is a positive instead 

 of being a negative, and consequently 

 the paper prints from this are nega- 

 tives and not positives. 



It would be ^'ery easy to overlook 

 this difference in a large class of 

 micro-photographs, because, in an 

 alternation of dark and light lines, or 

 dark and light spaces, it often matters 

 little which of a pair is light or dark ; 

 the picture may be equally clear and 

 satisfactory either way. In the case 

 of a large majority of the microscopic 

 objects photographed, either the posi- 



• Read before the American Microscopical Society. 

 Revised by the Author. 



