aronnd the vertical axis carrying all that is above along with it. The second 

 arm, supporting the view telescoi«, swings around independent oF the collimator 

 and circle but carrying witli it tiie reading microscopes whieh are attached to it 

 by a strong cross beam at right angles. Tiie circle has a movement independent 

 of everything below it and the table npon which the grating is placed also rotates 

 independent of the circle. The position of this table may be resul, wlien necessary, 

 by means of two verniers placed ojiposite to each other. All of the movements 

 are, of course, provided with the necessary clamp and tangent screws. It will be 

 observed that the arms supporting the view and collimating telescopes are 

 provided with balancing weights so that both ease of motion and staljility are 

 secured. The eye piece of the view telescojKj is provided with a micrometer, as 

 is also the collimator for measuring and adjusting the width of the slit. A 

 striding level is provided for the telescope and collimator and levels are also 

 attached to the arm of the collimator and to the grating table. The latter has 

 screws for leveling and for centering. 



The instrument was mounted upon a stone ])ier in a small building erected 

 for the purpose, the inside walls of which were blackened. The direction of the 

 collimator was nearly North and South. Through a small opening in a shutter 

 opposite the collimator a In-am of light was thrown from a heliostat mounted upon 

 a pier just outside. 



ADJUSTMENTS. 



It will be seen that all neces.^ary adjustments can be made with little 

 difficulty. The axis of the instrument is made vertical by means of the levels. 

 The telescojMj and collimator are adjusted for height and direction by means of 

 the vertical pillars ujion which they rest, which have movements in a vertical 

 direction and about vertical axes. The telescope and collimator and the grating 

 table are then leveled. The adjustment of telescope and collimator for parallel 

 rays is easily accomplished by means of the collimating eye-piece, using the sm-- 

 face of the grating as a reflector. The advantage in using this form of eye-piece 

 in this work is very great. Each telescoj« is adjusted to its focus independent 

 of the other and when it is desirable to render either telescope or collimator 

 normal to the grating it is accomiilished accurately, quickly, and without the aid 

 of the circle. The slit and cross wires are adjusted to the line of collimation by 

 turning the telescopes in their supporting Y's. The face of the grating is 

 rendered vertical by means of adjusting screws arranged for the purpose, using 

 the reflected image of the cross wires. To make the lines of the grating vertical, 

 a diaphragm is placed over the slit so as to reduce greatly its length. A very 

 narrow siiectrum results and it is easy to adjust the grating so that this appears 

 in the same position on both sides of the collimator. 



The method adopted for malcing the observations, being that of normal 

 incidence, is di.scussed in detail in another place. A setting was made upon the 



