1871.] 119 
[Heller and Brightley. 
every case to be indistinct and not in focus ; onthe contrary, focus it soas to 
be distinct at the edge, and it will be indistinct when brought to the centre. 
In some telescopes, however, it is impossible to focus at the outer edge 
of the field, and objects will be tinged with prisniatic colors, showing 
that these glasses are affected by chromatic aberration also; sometimes 
the cause of this defect lies in the object zlass, but in the majority of 
cases, the lenses composing the eye-piece are in fault. 
These aberrations affect the working of the telescope in several ways. 
First, it practically diminishes the size of the object glass, and the view 
is never so clear and distinct as it ought to be. Second, It is very diffi- 
cult, and in some cases almost impossible, to adjust the eye-piece to 
prevent parallax, or “‘travelling”’ of the cross wires, when the eye is 
shifted from side to side; and practical engineers know what a sharper 
power of defining, and how much less trying to the eyes a ‘‘soft glass” 
has—that is, one that has a ‘ flat field.’ 
This defect has prevented the general use of the Stadia, or Micrometer 
Wires, as a method of measuring distances without a chain, as the two 
horizontal hairs that are used, being in diffrent parts of the field of view, 
can not, in a majority of cases, be focused so as to be devoid of parallax, 
and the slightest travelling of the wires in this operation will give an 
erroneous result. 
The evils of this defect were most forcibly brought to Mr. Heller and 
the late Wm. J. Young’s notice, when one of their best Transits failed to 
define in tunnel work, from loss of light, from this cause, and they both 
endeavored, to within a short time of Mr. Young's death, to remedy it, 
trying all the known formulas of almost all the opticians in the country, 
but without any good results. 
In the Telescope of this instrument these evils are entirely removed, by 
the employment of a new eye-piece, and advantage has been taken of - 
the improvements that Optics have made in the last few years, in the 
curvatures and arrangements of the lenses that compose it; and the test 
referred to above, of focusing an object in the centre of the field of 
view, and then bringing the same object to the edge, and it still remain- 
ing in sharp focus, can be done with this telescope, and the object shows 
no tinge of prismatic color, showing that both chromatic and spherical 
aberration have been removed. 
The advantages of this improved Telescope are : a clear and sharply de- 
fined field of view ; a field of view so flat that the cross hairs are without 
parallax in every part of it, and micrometer hairs or Stadia can be used 
with favorable results. 
The whole effective power of the object glass being used and none of 
the light lost, work can be commenced earlier in the morning and contin- 
ued later in the afternoon than is usual. This, in the winter season, is 
