344 Dr. L. Bleekrode. On the Determination of the 



Suppose in fig. 3 a liquid layer of a certain thickness limited by 

 parallel surfaces, its refractive power shall be such that any object 

 placed in b will be seen apparently in c ; hence the relation 



(Jb tan i sin i _ 



dc tan r sin r 



when the angles of incidence and refraction are very small, calling 

 the thickness of the layer D, and the apparent displacement of i=d, 

 the equation is obtained 



D 



n= 



D-d' 



which gives the value of the index by measuring two linear 

 dimensions. 



Practically the case is not so very simple as is supposed, because 

 the terminal glass plates, transmitting the rays, contribute to the dis- 

 placement, as is shown in fig. 4. The effect caused by these two 

 disks together I call the " glass effect," and it had to be determined 

 for each vessel itself before it was filled with liquid ; the amount was 

 afterwards deducted as a constant correction. In fig. 4 the vessel is 

 represented by abdc, and it is obvious that the rays, which can be 

 transmitted through the liquid under the greatest angle of refraction, 

 have a direction along ac, when they will pass through the apertures 

 in the centre of the steel plates laid on ab and cd. 



In the greater part of experiments, ab and cd measured 20 and 21 

 millims. respectively ; the angle of refraction deduced from its tangent 

 will not surpass a value of 5|, and therefore the error, caused by 

 taking the sine instead of the tangent, only amounts to one unit in 

 the third decimal. 



The numerical values for D and d are easily obtained with the aid 

 of the microscope, which was specially constructed for my purpose 

 by Nachet of Paris. The ocular tube (that in most observations was 

 placed in a horizontal direction) was provided with a vernier, moving 

 along a scale divided in halves of a millimetre, and allowing to deter- 

 mine a displacement of the tube to millim. ; a micrometer screw 

 could also be used for a more accurate adjustment to the -^Q millim. 

 Of course, the exactitude that may be reached in focussing any object 

 depends on the magnifying power applied ; unfortunately, I was 

 limited in using very high powers by the necessary dimensions 

 required by the glass cell to be strong enough and fit to the end in 

 view. I chose a high magnifying ocular power and a very feeble objec- 

 tive lens, getting by this combination a magnifying power of nearly 3C ; 

 it gave the advantage of having very well-defined and clear images 

 to focus on of microscopic test-objects as, for instance, the scales of 

 wings of Lepidoptera fixed on the glass plates of the cell merely by 

 adhesion on the inner surfaces. When the cell was constructed, I 



