\ 
TRANSACTIONS OF THE SECTIONS. 15 
other vils) was stich as to fefider the line H very indistitict; its index is therefore 
Marked as doubtful: 
p for the standard rays. 
Medium, B. σ. D. E. F. G. H. 
Oil of Spikenaid ψ ἢ : κ i ; él Z 
temp. 22° ἀν} 1:4732 [1.4740 11.478 11.4829 | 1-4868 ]1.4944 | 15009 
Oil of Sandal-wood, || ,, : ἢ ; ; : i 3 
τοίη. 20°...6:..4: } 1:5034 | 1°5058 | 1:5091 | 1°5117 [1.515] | 1°5231 | 15398: 
Oil of Lavetder, 2811 112 {8 ξ 11:5 aT 4937 | 149302 
temp. 20°... 1-4641 | 1-4658 | 1-4660 | 1-4728 | 1-4760 | 1-4837 | 1°4930: 
Benzole, temp. 189... 1-4895 | 1-4961 | 1-4978 | 1.504] | 1:5093 | 15206 | 15310 
In my report (1839) I stated the impossibility of obtaining measures in chromate 
of lead from the absence of all appearance of lines, and the entire absorption of the 
blue and violet portion of the spectrum. I have since thought that in the abserice 
of any determinations of the kind it might not be useless to give the very rough esti- ὃ 
mates whith my former attempts enable me to obtain by means of the absorption of 
blue glass; which gave a point roughly corresponding to about B, another to D; and 
the extreme green space visible might be about E. The most refracted of the two 
spectra (given by the double refraction of the substance) was the worst defined ; and 
in this the part corresponding to D is extremely uncertain. The mean of two sets 
of observations was as follows :— 
Prism of chromate of lead; axis of prism perpendicular to axis of crystal; mean 
angle obtained by reflexion and by measurement=14° nearly. 
1st Spectrum. 2ud Spectrum. 
Ray. A. μ- Δ μ- 
Extreme red about B...... 22 | 2°53 26 30 2°84 
a about Ὁ ..... 93 10 | 2°55 39? 30 
᾿ about E....| 94 30 | 2°70 30 30 | 3-10 
While upon the subject 1 may be allowed to rethark, that a8 attempts are now 
imakitig,; with so nitich promise, for procuring optical glass of a stipérior quality; it 
wotild be highly ititerestitig if specimens were cut itito ptisitis (portions of 2 aii inch 
cube; or even less; will do, and two sides only need be polished; containing an atigle 
of about 60°); so ds th siibject the glass ἐδ the very delicate test of the visibility of 
the finer lines of the spectrum. 1 have reason to think that working opticians dre 
not generally aware that in many Specimens, apparently very clear, only afew of the 
broader lines cati be sten, dnd very often none ; whereas in Fraunhofer’s glass nearly 
600 were visible. 5 
Erratum in the Table, Brit. Assoc: Report, 1839; p: 10. 
Nitric avid, Ray G, for 1-4855 read 1:4155: 
On a New Solid Eye-piece. By the Rev. J. B. Reade, F.R.S. 
The author stated that he had been able to get rid of the two well-known defects 
of the common negative eye-piece; viz. a play of false light and the formation of a 
false image, or as it is generally termed; @ ghost of a planet or star, by simply filling 
the eye-piece with water. The addition of the water causes the ray of light to pass 
to the eye without suffering any inner reflexions from the surfaces of the lenses of 
the eye-piece. It also makes the eyé-piece positive instead of negative; while at the 
same time the magnifying power remains nearly the same; the magnitude and flat- 
