Mr* J. P. Cooke on a Spectroscope. 1 1 7 



extreme red, and the smallest diameter, 8 J inches, corresponding 

 to the extreme violet rays of the solar spectrum. In order to 

 facilitate the adjustment, a series of lines were engraved encir- 

 cling the wheel at equal distances from each other, and numbered 

 from 1 to 12. 



Having described the construction of the instrument, it will 

 now be easy to understand the method of using it. Let us sup- 

 pose that the object is to measure the angle of minimum devia- 

 tion of the blue ray of the strontium spectrum. By examining 

 any chart of the spectra of the chemical elements, it will be 

 found that this line is situated roughly at somewhat less than 

 two-thirds of the distance from A to H. If, then, we turn the 

 conical wheel until the pins of the prisms all rest against the 

 line marked 7, we shall have approximately the true position. 

 We then adjust the collimator with reference to the first prism 

 exactly as before described for a single prism. Turning then 

 the upper plate so as to remove the prisms out of range, we 

 bring the observing-telescope into collimation with the colli- 

 mator, as also before described, when, on reading off the limb, 

 we have the starting-point for our measure. We next turn the 

 plate and move the telescope until the spectrum appears in the 

 field, and carefully bring the blue line to coincide with the ver- 

 tical wire at the position of minimum deviation. We now 

 raise or lower the conical wheel and notice if in this way the 

 angular deviation is diminished, and leave the wheel in the posi- 

 tion where the minimum is reached. It now only remains to 

 again read the limb, when the difference of the two readings 

 subtracted from 360° will give the angular deviation required. 



When near the position of minimum deviation, a large motion 

 of the conical wheel produces only a slight motion of the image ; 

 so that, after a table has been made giving the position of the 

 wheel for a few of the marked lines of the spectrum, it is always 

 possible to bring the wheel at starting to the desired point. 

 Moreover the fact that, when near the position of minimum 

 deviation, the position of the image is affected so slightly by a 

 small change in the position of the prisms renders it possible 

 to make all the adjustments required with exceeding rapidity and 

 accuracy. 



In order to test the accuracy of our method, we have made 

 several determinations of the minimum deviation of the line D ; 

 and although between each determination the whole apparatus 

 was completely dismounted, the value obtained was in all cases 

 267° 37' 50"*. It will be remembered that the sum of the angles 

 measured on each prism separately at the centre of the plate, as 



* As the mean of the two lines. 

 Phil. Mag. S. 4. Vol. 31. No. 207. Feb. 1866. K 



