1883.] 



MICROSCOPICAL JOURNAL. 



33 



All my experience goes to prove they 

 are wrong. 



Since commencing this investiga- 

 tion I have received from Mr. 

 McNeil, specimens of diatoms from 

 Pensacola and Mobile Bays, and ex- 

 pected to add the investigation of 

 them to this paper; but 1 find the 

 Pensacola, while containing most of the 

 Tampa Bay species, has also many 

 others that I need more authorities for 

 consultation, and more time for their 

 study. 



Photography and its Talue in 

 Microscopical I n vestigations. 



Last year the process of making 

 photographic illustrations of micro- 

 scopic objects was quite fully de- 

 scribed and commented upon. The 

 introduction of dry plates, all ready 

 for exposure to the image formed by 

 the microscope, and much more sensi- 

 tive to light than the wet collodion 

 plates used in the past, has enabled 

 the microscopist to arrange an appara- 

 tus for photographing specimens at a 

 very slight expense, using an ordinary 

 kerosene lamp as the source of light. 

 In view of these facts, it is a matter 

 of interest to inquire whether photog- 

 raphy affords a means of illustration 

 or demonstration in any wise equal or 

 superior to drawing by hand. On the 

 one side it may be said, that a photo- 

 graph is necessarily a faithful and 

 absolutely correct representation of 

 the object. This may be true and it 

 may not be true. Ordinarily it is so. 

 But somewhat depends upon the na- 

 ture of the object. A transparent 

 object does not appear the same as an 

 object shown by reflected light, and it 

 will not be reproduced the same upon 

 a photographic plate. The color of 

 the parts influences unequally the 

 actinic power of the transmitted light. 

 Thus, in an insect preparation, the 

 yellow chitinous portions obstruct the 

 most active rays of light. In order 

 that the detail observed in these parts 

 by the eye shall be impressed upon 

 the sensitive plate, a rather longer 



exposure is necessary than for the 

 other parts. The dry plates, however, 

 are far more sensitive to rays of yel- 

 low light than the wet plates hereto- 

 fore commonly used, and they will 

 give better pictures than the latter. 

 Siill, there is a loss of detail in many 

 preparations because of the absorp- 

 tion of actinic rays by certain portions 

 of the objects. 



Nevertheless, photography affords a 

 very useful method of delineating 

 microscopic objects, and it is a very 

 rapid method. If the apparatus is 

 ready for use, a perfectly good photo- 

 graph can be taken in five or ten min- 

 ute^s, by lamp-light. 



On the other hand, it may be said, 

 that the photograph only clearly rep- 

 resents what is in focus at one time, 

 while the observer studies, and gets 

 the relation between, different planes 

 by moving the focusing screw back- 

 ward and forward. Hence a pencil 

 drawing more truthfully represents 

 an object as it appears to the miud of 

 the observer. This is undoubtedly a 

 fact, and for this reason there can be 

 no doubt of the superior value of the 

 drawing. Yet drawings require a 

 much longer time for execution, and 

 their excellence partly depends upc n 

 the skill of the artist, and partly upc n 

 his familiarity with the use of the mi- 

 croscope. 



We are inclined to believe that 

 both the photographic and the free- 

 hand methods of illustration or dem- 

 onstration, have certain advantages 

 of their own. In spite of the minor 

 defects already mentioned, the pho- 

 tograph bears evidence \ipon its face 

 of the accuracy of the observations 

 which it is relied upon to sustain. It 

 is to be regretted that photographic 

 representations of objects cannot be 

 demanded to settle disputed points. 

 It would be an invaluable aid to Dr. 

 Carl Heitzmann, for example, if he 

 could photograph the blood-corpus- 

 cles in the various phases he hsa 

 represented in his book, " Micros- 

 copical Morphology," on pp. 68-9 the 

 like of which we are quite sure was 



