176 THE MICROSCOPICAL NEWS. 
pictures taken with a Beck’s one-fifth are much clearer though 
smaller than the plate taken with Zeiss’ one-eighteenth. There is 
more detail in the latter, and here comes in the value of amplifi- 
cation and aperture. 
III. We have not yet noticed the third element which goes to 
make up penetration in the microscope. The depth of vision 
increases in direct proportion with the refractive index of the 
mounting medium. Thus if a combination of lenses possessed a 
penetrating power of 100 micras when used over an object mounted 
dry that depth would be increased to 133 » when mounted in 
glycerine, and to 152 » in balsam, 168 yp if in iodide of potassium 
and mercury, and to 210» when it is mounted in phosphorus. 
The great gain in stereoscopic effect, on objects mounted in a 
medium of high refractive index, has led Mr. E. Ward, of Man- 
chester, to mount opaque objects in balsam, with extremely good 
results. 
FISH CULTURE AT WALKER WOOD 
RESERVOIR. 
By R. Sranzey, Esq., J.P. 
DARE say many of you think it is scarcely necessary to take 
this trouble, as the fish would naturally breed themselves. 
So they did before these reservoirs were made, when the stream 
was in its normal condition; but in making these reservoirs the 
natural breeding places were covered with deep water in which 
trout spawn would be destroyed when deposited by the fish. 
Trout spawn, to act successfully, must be placed on a gravel 
bottom, in pure spring water, from one to three inches in depth ; 
but even then a very few of the eggs hatch out, as birds, fish, and 
many kinds of larve eat great numbers of them, leaving very few 
to grow to mature fish. If therefore we did not resort to artificial 
breeding scarcely any fish would be found in the reservoirs. 
In artificial breeding, however, we can increase the number to 
any extent, as, on the average, trout or any of the salmon tribe 
will yield 1,000 éggs to each pound weight of the parent fish, of 
which we can hatch out over go per cent. 
Very pure water is only required for hatching purposes, as 
mature fish will live and thrive in comparatively impure water, 
provided that their natural food is present ; so that there are a 
great number of ponds, brooks, and canals that would maintain 
fish, but which are not adapted for breeding fish. 
