ZOOLOaY AND BOTANY, MICROSCOPY, ETC. 581 



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 nature of the object. A trans- 

 parent object does not appear the same as an object shown by reflected 

 light, and it will not be produced the same upon a photographic 

 plate. The colour 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 neces- 

 sary than for the other parts. The dry plates, however, are far more 

 sensitive to rays of yellow light than the wet plates heretofore 

 commonly used, and they will give better pictures than the latter. 

 Still, there is a loss of detail in many preparations because of the 

 absorption of actinic rays by certain portions of the objects. 



On the other hand, it may be said that the photograph only clearly 

 represents what is in focus at one time, while the observer studies and 

 gets the relation between different planes by moving the focussing 

 screw backward and forward. Hence a pencil drawing more truth- 

 fully represents an object as it appears to the mind 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 upon 

 the skill of the artist, and partly upon his familiarity with the use of 

 the Microscope. 



Both the photographic and free-hand methods have, in fact, 

 advantages of their own, the photographis, however, furnishing 

 evidence of the accuracy of the observations which it is relied upon to 

 sustain. 



Abbe's Refractometer.* — Since the introduction of homogeneous- 

 immersion objectives it has become a matter of increasing importance 

 to be able to readily determine the refractive index and dispersive 

 power of a fluid, without having to resort to the old cumbrous 

 methods by hollow prisms, &c. The refractometer devised by Pro- 

 fessor Abbe enables this to be done with a facility and accuracy that 

 leaves nothing to be desired. 



The leading principle of the apparatus depends upon the obstruc- 

 tion of the rays by total reflection at the surface of the fluid under 

 examination. WoUaston and others have previously adopted the 

 method of observing the maximum intensity of the reflected ray, but 

 a great advantage is gained by observing instead the minimum 

 intensity, so to say, of the transmitted ray. In the former case there 

 is a difficulty in ascertaining the precise point when the light reaches 

 its maximum, whilst in the latter a very small amount of light is 

 easily detected in the darkened field. 



The principle will be better understood by reference to fig. 113. 

 Two similar prisms A and B of highly refracting flint glass, with 



* Abbe, E., 'Neue Apparate zur Bestimmung des Brechungs imd Zer- 

 streuungs-vermogens fester und iiussiger Korper,' 8vo, Jena, 1874, 79 pp. (1 pi. 

 and 7 figs.). 



