TRANSACTIONS OF THE SECTIONS. 
3 
2. When the incidence is oblique, the ray refracted in this medium emerges from 
it in a direction parallel to that of the incident ray, and the distance included between 
ila sc two directions, or the transport, only depends on the angle of incidence, on the 
judex of refraction of the refractive medium, and on the thickness of this medium. 
From the relation subsisting between these four quantities we may therefore deduce 
the index of refraction, if the three others ho known; the incidence may be chosen 
it pleasure, the thickness is easily measured, and the transport, the only unknown 
quantity, is determined by observation. 
Let a be the angle of incidence, p the angle of refraction, c the thickness of the 
medium, and n the index of refraction for a ray which traverses the plate J d the 
distance sought, then 
n _sin« ta etangp+-———« 
° 2 cos et 
sin p 
hence 
. . / 2 e coset \ 2 
«=sm(*V 1-H-—=->/• 
Y2fl sin m —«/ 
This formula will give », when e being measured and a given, d lias been deter¬ 
mined experimentally. 
file Apparatus which serves for the determination of d is composed— 
>• Of a sight Termed by a spiders thread fixed vertically in the interior of a lion- 
tom.ii tube; at one extremity of this tube is a diaphragm, through the aperture oj 
"'uch the light penetrates into the apparatus; a lens placed between the sight and 
' u ‘ 'taphrugui, and capable of motion in the direction of the uxis ol the tube, allows 
u>« light received to be conveniently regulated. , . „ . « 
- (It u horizontal circle for the determination of the incidences; at ® ’ 
1 ’circle there is a support which tnav be turned by un alidade traversing the limb 
the circle. 
Of» telescope furnished with two very fine wire* crossing in the focus of die eye- 
,■ "* "P tic uxis is parallel to tlmt of the tube containing the sight, and it 
' ,|)al,le motion in a direction perpendicular to this axis. The amount of motion 
’ un<1 cliecled by means of a mieromctric Bcrew. . ., 
! ,J —'l’he index of the alidade is placed so as to coincide With tne 
11 the circle; the telescope is pointed towards the sight, and the la horizontal 
UM f ?V . tl,e l,ble ia vertically against a frame placed on t he hor *onM 
'h- ?PP° rt - 1111(1 hy turning it on its axis, without disarrange - ' ivc d 
t *. on which it should retain, which iB Buch thut the sight may P 
Tf, !i , m . tirttl > may be found in a few seconds, , , . „„ nnrfe — a. 
^ d ' tlle a '‘dade ls turned to an angle a, and then to r jf rac ted 
i,„ , ' ,u *i-'scope so as to point it upon the sight in both positions of t , 
»4 „ l' 1 ** tfcrJr^n by tho optic axis of the ij-w-gy 
from a 7, e ' v ' 8 1VCS the value of d. The very trilling error. » hw “ Jg ray3 
i. 1V j obliquity of the plato in relation to the direction ot lll( ', i tic '\ t 
„ i^pentated in .he following manner ;-lf the plate be rittty i 
**<25 T ,u a5Us 18 ° n . ‘he distance of the emerging P ! ‘'' u ' el ,a) 0 ^ na - 
I H , ftVera ge of the values thus obtained will give a^ “‘°* f “ P ^|l e lism 
,n ol u ' tanc, ‘ s | n,1 ght. I have ascertained positively that a w cxact j. 
Ill-Jo ii^Xmar ' en Very 8a,u,ible h y tllc *pheroraeter, does not afh 
hmu receive either the rays of the solar spectrum, 0* the of white 
J suitable absorbents/or i, may be illuminated by 
‘In.,, v ^“ ft J n e average index of refraction of the refractive body is obtain , 
" h - ^vTV ,lay bl ' mea3Urc d by means of any spherometer. hut in^l.B 
* cr **» in tho^wo ^ ctannilie the relation between the lengths of 
C!cl 't to (] L . (rr „ 0 ‘UStrumetita, which is not attended With any di \ 
:■ 4SSP ! hC Va,Ue ^ 4 ‘or a substance of which the «l*£o ’£ r ^ing 
[ T, i.ula. pi - w hnown ; the value of c may then be deduced fr .geometer, 
b,t [ hrir re |:!tinn Ua ','. tlty wil1 usually differ from that furnished with the 
’ 111,6 nicety v 1 Wl11 always enable the values of c and d to be ® P j given 
y ‘ Ne '-«theless, in order to avoid this correction, M. J• I*”*** e 
