WOODWARD, ON NOBERt's TEST-PLATE. 229 



years ago, a ^th, an immersion -^th, and a -^-'-^th, by Wales, 

 &c., I obtained, the best results with the Tryth and ^th of 

 Powell and Lealand. In illuminating the object I found it 

 best to use the larger diaphragm opening of the achromatic 

 condenser without any central stop, and to give obliquity to 

 the pencil by throwing the condenser to the right or left of 

 its true centreing. AVith this management and both of the 

 above-named lenses, I at first supposed I had seen the lines 

 of all the bands, including the 19th. On attempting to 

 count them, however, with a good cobweb micrometer made 

 by Stackpole, of New York, I found myself unable to get 

 beyond the 9th or 10th band, on account of the tremor com- 

 municated to the instrument when the micrometer screw 

 was turned. This tremor, almost imperceptible with a -ith, 

 appeared so considerable with a Vr^h as to render an accurate 

 count impossible. Under these circumstances, I requested 

 my able assistant. Dr. E. Curtis, to undertake the prepara- 

 tion of photographs of each of the bands. This he did with 

 the -J-th, and a distance which gave as nearly as possible 

 1000 diameters. 



The photographs showed that the true lines had been seen 

 up to the fifteenth band inclusive ; those seen in the last four 

 bands were spurious. A subsequent count of the lines in 

 the last four bands, by the method to be detailed hereafter, 

 verified this opinion. A photographic trial of the J^yth on 

 the twefth band did not give so sharp a picture as that of the 

 same band obtained by the V-th, probably because the cover 

 was somewhat thick for this glass, for on Podura, with a suit- 

 ably thin cover, the yV^^^ l^^s excelled the Vt^Ii in our hands. 



The series of photographs thus obtained give the following 

 count for the lines in each band : 



The photographs of these bands present the following cha- 

 racteristics : — In the first band, the space immediately bor- 

 dering each side of the broad, deep, black lines is brighter 



