Actual Thickness of Microscopic Covering Glass. 271 
sured by the Swiss gauge their actual thickness ; secondly, the 
optical thickness ; and thirdly, the full thickness, reading both by 
the fine adjustment divisions. Tabulated results in the three 
cases : — 
Cover I. Cover II. Cover III. 
Actual thickness 6" (thousandths) 4" -6 .. 2 " I * * 4 2 
Image reading, fine adjustment .. 9 divisions.. .. 6''-9 .. 3"-3 
Full thickness, „ .... 19 „ .... 14 ..7 
The average reading by the table previously calculated would 
have been considerably at variance with these results. In my 
Swiss gauge the image reading is just half as much more than the 
thickness in thousandths or the actual thickness measured by the 
divisions in the fine adjustment at the rate of 3 divisions to 2 
thousandths of real thickness. 
Some interesting facts here appear : that the optical thickness 
measures the actual for the same kind of glass in every thickness 
of cover ; and that the refractive index is so high in this kind of 
glass that the image rises considerably more than one-third of the 
thickness, as commonly stated loosely enough, on the assumption 
that the index of refraction is 1 , 500. 
Sir John F. Herschel gives the following measures of the rays 
of mean ref raction of different kinds of glass : — 
Plate glass . . . . 1 • 500 to 1 • 540 
1-563 
1-642 
For the line D in the Spectrum, 
or for mean rays. 
Crown 
Flint „ 
Glass lead 
Flint 
Glass lead 
Sand 
Glass lead 
Flint 
?} 
!} 
1-525 „ 
1-576 „ 
1-732 
1- 987 
2- 028 
I have frequently asked opticians who supplied me with the 
best glass covers, what was the nature of the glass, but could get 
no answer. I believe it is flint glass. Messrs. Chance and Co., 
of Birmingham, could tell us what its composition is, and give us 
perhaps information as to its refractive index. 
