O P T 
sates in a locket y, that turns oil the axis X; both arms 
are prevented from bending by the braces aa. GE re- 
prefents a double fcrew, having one part E cut into dou¬ 
ble the number of threads in an inch to that of the part 
G ; the part E having 100 threads in one inch, and the 
part G 50 only. The fcrew E works in a nut F in the 
Tide of the telefcope, while the part G turns in a nut H, 
which is attached to the arm B; the ends of the arms B 
and D, to which the mirrors are fixed, are feparated from 
each other by the point of the double fcrew prefiing againft 
the ftud />, fixed to the arm D, and turning in the nut H 
on the arm B. The two arms B and D are prefied againft 
the direftion of the double fcrew' EG by a fpiral Ipring 
within the part 11, by which means all (hake or play in 
the nut H, on which the meafure depends, is entirely 
prevented. 
From tlie difference of the threads on the fcrew at E 
and G, it is evident, that the progreflive motion of the 
fcrew through the nut will be half the diftance of the re¬ 
paration of the two halves of the mirror; and, confe- 
quently, the half-mirrors will be moved equally in con¬ 
trary directions from the axis of the telefcope C. The 
wheel V, fixed on the end of the double fcrew, has its cir¬ 
cumference divided into 100 equal parts, and numbered 
at every fifth divifion with 5, 10, &c. to 100, and the index 
I (hows the motion of the fcrew with the wheel round its 
axis, while the number of revolutions of the fcrew is 
ihown by the divifions on the fame index. The fteei 
fcrew at R may be turned by the key or handle S, and 
ierves to incline the (mail mirror at right angles to the 
direftion of its motion. 
Beiides the table for reducing the revolutions and parts 
of the fcrew to minutes, feconds, See. it will require a table 
for correcting a (mail error which arifes from the eccentric 
motion of the half-mirrors. By this motion their centres 
of curvature will approach a little towards the large mir¬ 
ror: the equation for this, purpofe in fmall angles is in~- 
fenfible; but, when angles to be meafured exceed ten mi¬ 
nutes, it fnould not be negleCted. Or, the angle mea¬ 
fured may be corrected by diminilhing it in the propor¬ 
tion the verled line of the angle meafured, fuppofing the 
eccentricity radius, bears to the focal length of the fmall 
mirror. Mr. Ramfden preferred Caffegrain’s conftruc- 
tion of the" reflecting telefcope to either the Gregorian 
or Newtonian ; becaufe, in the former the errors of one 
fpecujum are corrected by tbofe of the other. From a 
property of the reflecting telefcope, not generally known, 
that the apertures of the two fpecula are to each other 
very nearly in the proportion of their focal lengths, it 
follows, that their aberrations will be in the fame pro¬ 
portion ; and thefe aberrations will be in the fame direc¬ 
tion, if the two fpecula are concave; or in contrary di¬ 
rections, if one fpeculum is concave and the other con¬ 
vex. In the Gregorian telelcope, both fpecula being 
concave, the aberration at the feconcl image will be the 
fum of the aberrations of the two mirrors; but in the 
Caffegrainian telefcope, one mirror being concave and the 
other convex, the aberration at the fecond image will be 
the difference between the two aberrations. By affuming 
fuch proportions for the foci of the fpecula as are gene¬ 
rally ufed in the reflecting telefcope, which is about as 
1 to 4, the aberration in the Caffegrainian conltruCtion 
will be to that in the Gregorian as 3 to 5. 
The other inftrument invented and deferibed by Mr. 
Ramfden, is a dioptric micrometer,or one fluffed to the prin¬ 
ciple of refraCtion. This micrometer is applied to the erect 
eye-tube of a refraCting telefcope, and is placed in the con¬ 
jugate focus of the firlt eye-glafs : in which poiition, the 
image being conliderably magnified before it comes to the 
micrometer, any imperfection in its glafs will be magni- 
fide only by the remaining eye-glafles, which in any tele¬ 
fcope feldom exceeds five or fix times. By this pofition 
alfo th.e fize of the micrometer-glafs will not be the j£ g .th 
part of the area which would be required if it was placed 
In the objeft-glafs 5 and, notwithffanding this great dif- 
I C S. 627 
proportion of fize, which is of fome moment to the prac¬ 
tical optician, the fame extent of fcale is preferved, and 
the images are uniformly bright in every part of the field 
of the telefcope. This micrometer is reprefented at fig. 6. 
A convex or concave lens is bifeCted by a plane acrofs its 
centre ; one of thefe femi-lenfes is fixed in a frame B, and 
the other in the frame E ; which two frames Aide on a 
plate H, and are preffed againft it by thin plates aa: the 
frames B and E are moved in contrary directions by turn¬ 
ing the button D. Lisa fcale of equal parts on the frame 
B ; it is numbered from each end towards the middle 
with 10, 20, &c. There are two verniers on the frame E, 
one at M and the otherat N, for the convenience of mea¬ 
furing the diameter of a planet. Sec. on both fides the zero. 
The firft divifion on both thefe verniers coincides at the 
fame time with the two zeros on the fcale L ; and, if the 
frame is moved towards the right, the relative motion of 
the two frames is ihown on the fcale L by the vernier M ; 
but, if the frame B be moved towards the left, the rela¬ 
tive motion is Ihown by the vernier N. This micrometer 
has amotion round the axis of vilion, for the convenience 
of meafuring the diameter of a planet, See. in any direc¬ 
tion, by turning an endlefs fcrew F ; and the inclination 
of the diameter meafured with the horizon is ihown on 
the circle g- by a vernier on the plate V. The telefcope 
may be adjufted to diftinft vilion by a fcrew, which moves 
the whole eye-tube with the micrometer nearer to, or 
farther from, the objeft-glafs, as telefcopes are generally 
made; or the fame efteft may be produced without mov¬ 
ing the micrometer, by Hiding the part of the eye-tube m 
on the part ?i, by help of a fcrew or pinion. Phil. Tran]’. 
vol. lxix. art. 27. 
Notwithftanding thefe improvements on micrometers, 
they are (till liable to many fources of error. The imper¬ 
fections of the wire-micrometer, (which was (till the mod 
correct inftrument for meafuring fmall angles,) when em¬ 
ployed to determine the diftar.ee of dole double (tars, 
have been ably pointed out by Dr.Herfchel in the Phil. 
Tranf. for 1782. When two ftars are taken between the 
parallel wires, the diameters mult be included. Dr. Her- 
fichel in vain attempted to find lines fufficiently thin to 
extend them acrofs the centres of the ftars lb that their 
thicknefs might be neglefted. The threads of the filic. 
worm, with fuch lenfes as he ufes, are fo much magnified, 
that their diameter is more than that of many of the ftars. 
Befides, if they were much fmaller, the deflection of light 
would make the attempt to meafure the diftance of the 
centres this way fruitlefs; for he always found the light 
of the ftars to play upon thofe lines and feparate their ap¬ 
parent diameters into two parts. Now, fince the fpurious 
diameters of the ftars thus included, are continually 
changing with the ltate of the air and the length of time 
we look at them, we are, in fome refpeCt, left' at an un¬ 
certainty ; and our meafures taken at different times; and 
with different degrees of attention, will vary on that ac¬ 
count. Nor can we come at the true diftance of the cen¬ 
tres of any two ftars, unlefs we know the femi-diameters 
of the ftars themfdves ; for different ftars have different 
apparent diameters, which, with a power of 227, may 
differ from each other as far as two feconds. 
The next imperfeftion arifes from a defleftion of light 
upon the wires when they approach very near to each 
other; for, if this be owing to a power of repulfion lodged 
at the furface, it is eafy to fee that fuch powers mull in¬ 
terfere with each other, and give the meafures larger in 
proportion than they would have been if the repullive 
power of one wire had not been oppofed by a contrary 
power of the other wore. Another diladvantage of thefe 
micrometers is an uncertainty of the real zero. The lealt 
alteration in the fituation and quantity of light will affect 
the zero; and a change in the pofition of the wires will 
fometimes produce a difference. To remove this diffi¬ 
culty Dr. Herfchel always found his zero while the appa¬ 
ratus preferved the fituation which it had when his cbfler- 
vations were made; but this introduces an additional 
obfervation.:, 
