Heller and Brightley.] 1 1 8 [May 5th, 
In this instrument we have an improved tangent screw ; that no matter 
how much the screw may wear by use, or time, will never get ‘‘lost mo- 
tion,’”’ but will instantly obey the slightest touch of the hand : this is 
effected by means of a long cylinder nut, from the interior of which two- 
thirds of the screw have been removed ; into half the recess thus left in 
the nut, is nicely fitted a cylindrical ‘‘follower,”’ with the same length of 
screw thread as the nut; this follower is fitted with a ‘‘key,”’ that pre- 
vents it turning in the recess, but allows motion in the direction of its 
length. A strong spiral spring is placed in the remaining half of the 
recess, between the fixed nut and the movable follower, and the spring 
has always tension enough to force the follower and fixed thread in con- 
trary directions, and thus to remove any ‘‘lost motion’? that may occur 
in the screw. It will be observed, that in this method the spring always 
remains in a state of rest, instead of clusing and opening, as has been 
the case in all other applications of springs, and which have been the 
cause of their failure. Tangent screws that have had as much as 10/ 
play, have been made to work entirely taut by this method. 
The mode of attaching the tangent screw to the plates in this instru- 
ment is entirely new—it is a miniature modification of the ‘‘ Gimbelling”’ 
of a ship’s compass, and allows the tangent screw, by its free swivel- 
ling, to be tangent to the plates in every part of its length, and thus 
never to bind. This tangent screw is also of value for sextants, astro- 
nomicalinstruments, &c.,where lost motion is detrimental, and a smooth, 
easy motion is required. 
In all instruments, the brass cheeks in which the three legs of the 
tripod play, are fastened to the lower parallel plate by a number of small 
screws, commonly twelve. When the legs wear in the cheeks, and be- 
come unsteady, the only method the Engineer has of tightening the legs, 
is by drawing the cheeks, in which the leg moves, by means of the bolt 
that passes through the leg; this, of necessity, draws the cheeks out of 
perpendicularity, and strains the small screws that bind the cheeks to 
the parallel plate so much, as frequently to loosen them. This source of 
instrumental error hardly, if ever, occurs to the Engineer, but very good 
instruments have been condemned as unsteady, when an examination has 
shown the fault to be the above. ‘This source of error can never occur in 
this instrument, as the cheeks and the parallel plate are made in one 
solid piece. 
But to come to the last and most serious evil. The effective power of 
the Telescope is impaired by spherical aberration ; that is, the field of 
view, as seen in the Telescope, is not a perfect plane or flat, but is 
spherical. To prove this, take an ordinary telescope, and focus it so that 
an object will be clearly defined at the intersection of the cross hairs 
or the centre of the field of view, then, by means of the tangent screw, 
bring the same object to the edge of the field of view and it will be found in 
