1885.] 



MIOROSCOPICAL JOURNAL. 



203 



ver chloride, bromide or iodide are 

 changed in some way, not very 

 clearly understood ; where the light 

 is strongest the cliange is greatest, 

 and where it is weaker the change is 

 proportionally less, and thus an image 

 is impressed upon the plate, known 

 as the invisible image. because it is only 

 brought into view by the subsequent 

 process of development. The image 

 is not, however, absolutely invisible, 

 for it can be seen on a plate that has 

 been long exposed. The change pro- 

 duced by the light is not entirely un- 

 flerstood, but the weight of evidence 

 indicates that it is purely a chemical 

 change. It is supposed to consist in 

 a partial separation of the bromine, 

 chlorine or iodine, as the case may be, 

 from the silver. This change once 

 effected is permanent, and plates may 

 therefore be exposed at any time, and 

 developed months afterward. 



Coming now to the subject of de- 

 velopment, it will suffice to say that 

 a developer is a solution which com- 

 pletes and intensifies the action begun 

 by the light. Thus, if we suppose 

 the silver bromide is partially decom- 

 posed by the light, having lost a por- 

 tion of its bromine, the developer 

 removes the remaining portion of the 

 bromine, leaving metallic silver, in 

 the form of a black deposit. The 

 quantity of silver thus reduced to 

 metal in different parts of the film 

 corresponds exactly to the intensity 

 of the action of the light at those 

 parts ; hence, wherever the lights are 

 brightest, we find the thickest and 

 most opaque deposit of silver, and in 

 those parts where there was least 

 light — in the deep shadows of the 

 picture — there may be no reduction 

 whatever. Thus it is that what is 

 light in the object is dark on the plate 

 and vice versa ; hence we designate 

 the plate a negative. 



The next operation, known as fix- 

 ing, consists in dissolving from the 

 film all the unchanged silver com- 

 f)ound that is still sensitive to light. 

 When this is done the negative has 

 only to be washed and dried. It will 



then consist of a film of gelatin re- 

 taining the metallic silver picture pro- 

 duced by light and development. 

 [ 7y be ciniti)iucd.~\ 



The Magnifying Power of an Incli 

 Objective. 



Seeking to answer the question at 

 the head of this note, reference was 

 made to two or three catalogues with 

 unexpected results. It must be un- 

 derstood that in all cases referred to. 

 the standard length of tube and a two- 

 inch eye-piece are supposed to be 

 used. 



So far as the data go. the inch ob- 

 jective has 'assigned to it magnifying 

 powers ranging from 46 to 55 diam- 

 eters. On computing the inch-value 

 of objectives of higher and lower 

 powe^-, the same data being used, the 

 difterences expand and the extremes 

 are 36 and 60 diameters. These 

 figures require no comment. 



The system of nomenclature recom- 

 mended by the committee on eye- 

 pieces of the American Society of 

 Microscopists will no doubt soon be 

 universally adopted ; why cannot :i 

 similar committee be got to settle the 

 standard value of an objective, which, 

 with standard length of tube, and a 

 two-inch eye-piece, shall have a cer- 

 tain magnifying power and be called 

 a one-inch ? To this standard it will 

 be necessary to fix limits of deviation 

 to meet the mechanical difficulty in 

 making systems of lenses with pre- 

 cisely the same magnifying power : — 

 from this standard inch, all other ob- 

 jectives can have their standai'ds cal- 

 culated and their limits of deviation 

 decided. 



These suggestions are made in the 

 hope that microscopy may be placed 

 in possession of standardized tools, 

 whereby its results may approach as 

 closely as possible to precision and 

 uniformity. Owing to the defective 

 nomenclature now in use, much con- 

 fusion and not a little nonsense is fre- 

 quently presented. 



In the following table the power of 

 the inch is taken at 50, supposing a 



