Maech 4, 1910] 



SCIENCE 



345 



advantageous to have one of the cross lines 

 made shorter than the width of the spectrum. 

 The disc is then so placed in the instrument 

 that this short line is vertical, and hence 

 parallel with the spectrum lines. Under such 

 circumstances, when this short vertical cross 

 line is placed over a bright spectrum line, 

 the latter is seen extending above and below 

 it, and the small dark ends of the cross line 

 being thus brought prominently in view, ma- 

 terially assist in marking the spectrum line 

 upon which they are placed. The horizontal 

 arms of the cross are, in this case, of no par- 

 ticular advantage in marking the spectrum 

 lines, but they facilitate the finding of the 

 optic axis of the telescope, and, where the 

 instrument is provided with an illuminated 

 scale, help to align the same. It is best to so 

 place the scale that one end of the short ver- 

 tical line reaches about the middle thereof. 



Various devices may be employed to fix the 

 disc in the spectroscope. If the instrument is 

 provided with a negative ocular, the disc may 

 be placed against the diaphragm, and held in 

 position by a spring wire. It is well in that 

 case to provide the ocular with a sliding eye 

 lens, which can be cheaply done by any good 

 brass worker. If the instrument has a posi- 

 tive ocular and a diaphragm in it, or in the 

 telescope tube, the disc may, as before, be laid 

 against the diaphragm, and if such is in the 

 telescope tube, focused by sliding the ocular, 

 or if that be fixed, the diaphragm may be 

 moved till the cross lines are in focus. 

 Where there is a positive ocular and no dia- 

 phragm, as is the case with some instruments, 

 the disc may be cemented to a brass ring of 

 proper diameter to fit snugly inside the tele- 

 scope tube, and the proper position having 

 been found, the ring can be so set that the 

 cross lines will be at that point. Each of the 

 above devices has been tried in this labora- 

 tory and found satisfactory, and others will 

 probably suggest themselves. 



It is true that such devices do not always 

 succeed in making the center of the cross and 

 the axis of the telescope coincide; but this is 

 the case in but few cross wire spectroscopes, 

 and, for that matter, a spectroscope is not a 



transit, and does not require such a rigid ad- 

 justment of the line of collimation as the 

 latter instrument. If the center of the cross 

 is at the center of the disc, and the disc fits its 

 tube snugly, the cross lines will be sufficiently 

 centered. Were an absolutely accurate ad- 

 justment of the line of collimation worth the 

 cost, it could be secured by inserting an ad- 

 justable ring at the proper focal point and 

 attaching the disc thereto. 



The same method of supplying cross lines 

 answers equally well for microscopical obser- 

 vations, either for goniometric, or for polari- 

 scopic work; in fact, it was from noting its 

 utility in such microscopic work, that the 

 idea arose of applying it to the spectroscopic 

 investigations. 



Several of the above-described discs have 

 been made for this laboratory by the Bausch 

 & Lomb Optical Co. and they have given per- 

 fect satisfaction. C. M. Clark 



•J NOTE ON SOME PENNSYLVANIA FISHES 



During the warm weather of 1908 and 1909 

 Mr. E. W. Wehrle, of Indiana, Indiana 

 County, Pa., made a number of collections of 

 fishes, amphibians and reptiles, from his vicin- 

 ity. As almost all animal life is either extinct 

 or rapidly becoming so in the main basin of 

 the Conemaugh River, possibly the following 

 list will be of use in partly recording a van- 

 ishing fish fauna. I take this opportunity to 

 thank Mr. Wehrle for his care in collecting 

 full series of specimens, besides notes and in- 

 formation relative to the former condition of 

 the fish fauna. Notropis photogenis and 

 Micropterus dolomieu are from Cherry Run 

 and all the others are from Two Licks Creek, 

 besides such other streams as may be men- 

 tioned after each. Ichthyomyzon concolor, 

 Salvelinus fontinalis, Campostoma anoma- 

 lum, also from Ramsey's Run; Pimephales 

 notatus, Ramsey's Run, Harris's Run, Cherry 

 Run and Marsh Run; Semotilus atromacu- 

 latus, Ramsey's, Harris's, Cherry and Marsh 

 Runs; Leuciscus elongatus, Ramsey's and 

 Harris's Runs; Notropis cornutus, Ramsey's 

 and Cherry Ru.ns; N. atherinoides. Cherry 

 Run; Ericymha huccata. Cherry and Ram- 



