A New Befractonieter. By Dr. Boyston-Figott. 297 
tance nearer to the eye of the observer equal to two-thirds of the 
thickness of the plate. Indeed if t be the thickness and /z- the 
refractive index, the displacement is generally 
- (i. e. t divided by 
If, therefore, an instrument could be devised which would with 
great accuracy measure the thickness of the refracting plate, and 
also at the same time the distance by which the image of points on 
its surface was displaced inwards by refraction, data could be 
obtained for determining the value of /x. 
The instrument has assumed its present form after many con- 
structions and reconstructions. I was led to consider this method 
of finding the refraction of glass by frequent accidents happening 
while using the sVth of an inch objective with the microscope, which 
by pressure destroyed or cracked the thin glass covers generally 
applied to protect the objects or " slides." Now, so to speak, the 
observer always in such a case really examines the elevated image 
of the object, raised about two-thirds of the thickness of this cover. 
By knowing, therefore, the refraction of the glass cover and its 
thickness, such accidents, so irreparable in many valuable objects, 
might be avoided. Means were sought to determine the index of 
refraction of such covers, frequently varying from the hundredth to 
the thousandth of an inch thick, an extra thickness once having 
been destructive to a most valuable objective. 
Hitherto the method of finding the refractive index has been 
by the use of prisms made of the material in question, and em- 
ployed in the form of a spectroscope. 
-As an example of the power of the instrument, some flint glass, 
nearly half an inch thick, marked B, gave on three trials 
(JL = 1-6626 
1-6626 
1-6621 
Mean = 1-6624 
A thin glass cover about one hundredth of an inch thick gave 
H = 1-5502. 
The optical equation for a plate of glass, 
t 
V = u -{ 
(where u is the distance of the object and v the conjugate focus), 
points out that when the object is on the surface, or u = 0, 
t 
