27-t 



KNOWLICDGE. 



JlI.Y, 1912. 



as may be seen from the four examples here ({ivcn. rimtcj- 

 Krapliers are in the habit of saving that the linear si/c of the 

 ima^e is proportional to the focal len^;lll of the lens. But this 

 is not the case, and if we measure corresponding diiriensions 

 of A and B, taken with the four and three inch len.ses 

 respectively, we find that the longer focal length has given us 

 the smaller picture. Moreover, their linear dimensions are 

 not in the proiiortion of four to three. The fact is, firstly, 

 that here normal conditions of everyday photography are 

 changed and we are now using the longer conjugate distance. 

 .■\nd this does not vary simply in proportion to focal length. 

 Nevertheless, the rule is simplicity itself. If ur ..ill t the 

 major conjugate,/ the focal length 

 of the lens and in the magnifica- 

 tion, the formula is c= (! + «;)/. 



To apply this we subtract /from 

 c and then divide what is left 

 over by /. For example : with 

 twelve-inch camera and four-inch 

 lens. Subtracting four fromtwcl\' 

 gives eight, and dividing this 1' 

 four gives us two magnificatioii 

 In the case of the three-inch leu . 

 taking three from twelve lea\( 

 nine, which divided by three give-- 

 three magnifications. Now an ;i 

 varies as the square of linear si/i 

 and exposure varies as area size. 



Putting matters in tabular form 



we see the whole thing at a glance. 



A. B. C. D. 



Focal length of lens 4" 3" 2" ;'" 



Magnification (linear) 2 3 5 14 



Area 4 9 25 196 



Exposure ... ... 4" 9" 25" 3.}' 



Examples A, B, C, and D (see 

 Figure 302) were made with 

 quarter plates cut in half, exposed 

 as in the above table, developed, 

 and printed together. 



Now comes the everyday ques- 

 tion of equivalent exposures with 

 different lenses, camera lengths, 

 and stops, but the same object 

 and lighting conditions. For 

 example: — (i) Camera length, 

 fifteen inches ; lens three-inch 

 focus; stop, /. 11. (ii) Camera 

 length, sixteen inches; lens, two-inch focus; stop, /.16. 



Ascertaining magnification in the way just mentioned, we 

 get four and seven respectively. Area ratios are, therefore, as 

 sixteen to forty-nine. With the same / value of stop in both 

 cases, this would also be the exposure ratio. But we propose 

 using /.ll in the first case, and /.16 in the second, so our 

 exposure ratio becomes sixteen to twice forty-nine. i.e.. 

 ninety-eight, or, say, one to six nearly. 



By putting matters in tabular form, the arithmetic steps can 

 be seen at a glance : — 



Camera length ... 



Focal length of lens 



Magnification (linear) 



Relative areas 



Relative exposure with /. H 



both cases 

 Exposures with diiTerent stops . 

 Approximate ratio 



The object is a Brazilian liana, trans-section. Plates — 

 Imperial N.F. Daylight reflected by substage mirror. 



It is important to note that in the first table the exposures, 

 measured in seconds, happened to coincide with the exposure 

 ratio numbers. But this is merely a coincidence in this 

 special case, where the nature of the subject and lighting con- 

 ditions suggested four seconds for the first, and so the other 

 exposure times necessarily agreed with the ratio numbers. 



"^^P 



Figure 303. -V Meseulcry 

 deeply-!itaiiicd iiuercelUil: 



stance between the cells of 

 the pavement epitheliutn covering one surface of the mesentery. 

 (Photographed under Bausch and Lomb I objective, No. lo ordin.iry 

 ocular Bellows, length 7 inches). 



15in. 



16 



16 



K. If.W). 

 1 



Klin. 

 J-in. 



49 



49 



98 (/. 16) 

 6 



Turning lo the second table we see the ratio numbers are 

 roughly one and six, but whether the exposures be one and six 

 secontis or one and six minutes, and so on, will depend on the 

 nature of the object that is being dealt with, speed of plate 

 and lighting. I want to emphasize the point that the above 

 considerations only give us relative and not actual times. 



The practical application of the matter is this. By a few 

 trials one can ascertain the exposure of, say, object P, under a 

 given set of conditions, as in example (i) in the second table ; 

 but we want to deal with object O under conditions (ii». We 

 have ascertained for the same object that the exposure ratios 

 are as one to six. If now we view the two different objects 

 under the microscope, or as a 

 ground glass image under precisely 

 identical conditions, we can make 

 a reasonably good guess as to 

 their required exposures. Let us 

 say, by way of example, that P 

 seems to require about one and a 

 half times that for y, under 

 precisely identical conditions, i.e., 

 P to Q as one and a half to one, 

 or three to two. But the table 

 for different conditions (i) to (ii) 

 says one to six. Therefore, P 

 under conditions (i). and Q under 

 conditions (ii) combines these 

 ratios ; which we get by multi- 

 plying three and one and then 

 two and six, or three to twelve, or 

 one to four. Knowing the appro- 

 priate exposure of one of our 

 objects the other is at once 



estimated. ,- ,, , ,.„ 



!■.(.. L.XMBKK 1. 



AN ALTERNATIVE SILVER 

 METHOD FOR DEMONSTRA- 

 TING THE CEMENT SUB- 

 STANCE IN PAVEMENT 

 EPITHELIUM. — The usual 

 method for staining the cement 

 substance of epithelium consists 

 in soaking the tissue in silver 

 nitrate and exposing to sunlight 

 until the tissue turns brown. 



This method has a drawback in 

 that it is dependent upon a fine day 

 for a successful result. The method 

 given below overcomes this dis- 

 advantage by doing without the exposure to light altogether. 

 .•\ frog's mesentery may be taken as a typical example 

 upon which to work. The method is as follows :— 



1. Pin out the mesentery, with its encircling loop of intestine. 



upon a cork ; then wash it once in distilled water. 

 1. Place the cork with the tissue undermost, in a half to one 



per cent, solution of silver nitrate for five minutes. 



3. Wash in distilled water to rid the tissue of excess of nitrate. 



4. Prepare a hydrokinone developer of the usual photographic 



strength, and dilute it with five times its bulk of water. 

 In this immerse the tissue and cork, taking care to keep 

 the tissue evenly covered with the fluid all the time. 

 When the tissue has changed from white to a light grey. 

 or light brown, remove it and wash very thoroughly to 

 clear it of developer. 



5. Next place the tissue in a five per cent, solution of hypo- 



sulphite of soda for fifteen to twenty minutes. Before 

 starting Part 5 the tissue maybe removed from the cork. 



6. Wash thoroughly ; place in a dish of water and cut away 



the intestine which encircles the mesentery. Dehydrate 

 the latter in alcohol, clear in clove oil. and mount in 

 Canada balsam. 



It will be noticed that the method is divided into six parts. 

 I find that the chief f.ictor in obtaining a good result by this 

 method consists in paying careful attention to Part 4. It is 

 essential that the tissue should not be over-developed. 



^ 



th.j V: 



