WELCKER, ON MICROSCOPIC MEASUREMENTS. 243 
of air was replaced by a fluid,—for instance, by glycerine 
introduced between the two plates of glass—the accurate de- 
finition of the two surfaces now required a movement of the 
tube corresponding to 372 notches. The real thickness, 
therefore, of the layer of glycerine, was to the apparent 
thickness as 552 to 372, or as 148 to 100. To ensure cor- 
rectness, I prepared a second glass cell, within which the 
stratum of air measured 1002 notches, and that of glycerine 
678,—7. e., in the same proportion to each other of 148 to 
100. The fluids enumerated in the following collection were 
all proved in this double way. The refractive power of solid 
substances was estimated in a similar manner,—that is to say, 
by measuring the apparent thickness of a section compared 
with the thickness of a stratum of air of the same actual 
thickness. 
TABLE, 
Showing the ¢rwe thickness of various substances whose apparent thick- 
ness = 100. 
pA nee A : ; : . 100 
Distilled water . : : : . 138 
Spring water : 4 : : == Tos 
Human blood-serum : : oy 139 
Albumen of Hen’s egg (fresh) , ‘ #2189 
Blood-serum of Lacerta muralis « A LAO 
Human corpus vitreum (eighteen hours after death) . 140 
Sulphuric ether. : : : 24d 
Alcohol, 87° : : af ; . 142 
Recent muscle of frog > : : . 1426 
Water-glass of Batka (fluid) . 145 
Mucilage of gum (1 centigram. water ; ; 0-5 gramm, gum- 
arabic) : : : : . 147 
Glycerine . 148 
Thicker mucilage. of gum a centigram, water 5 1 gramm. 
gum-arabic) . «yl 
Water-glass of Batka (dry) . J an JDO 
Marrow (knochenfett) blenclied i in the sun : «1 bel 
Oil of turpentine . : : : >. au 
Oil of lemons ; : : : ~ 1515 
Human fat : : : ; a Lb? 
Rape oil : : : » 152°5 
Canada balsam (fresh) é , , «  Lb4°5 
Common crown glass ; : : 155, 156 
Copal varnish (fresh) : : : 22156 
Oil of aniseed : , : . 158 
Dry albumen : ’ Q : - 165 
Gelatine (dry). . : : » 70 
Bone . ; é : : ee 
Ivory . ; : b pl lits 
Enamel of Horse’s tooth : : ae | 


downward rnovement of the screw, the under ones ; because if the contrary 
direction had been followed, the spiral elevating spring might possibly not 
be exerted uniformly, and a dead turn of the screw-head be made, 
