2875.) Physics. 277 
owing to the neglect of a few necessary precautions, failures are common. 
The tank, with its rockwork, should be filled with fresh water, and all the 
soluble lime salts in the cement used in attaching the pieces of stone soaked 
out; this may take a week or two, and must on no account be hurried. After- 
wards the sea-water, natural or artificial, may be introduced,* and some ponds 
of Ulva or other suitable seaweeds placed in the tank, the object being to pro- 
mote the growth of minute alge on the surface of the rockwork. A seaweed 
already growing, although attached to stone, cannot be trusted to maintain 
the necessary supply of oxygen, as it but rarely establishes itself and forms 
vigorous plants. The rock should consist of sandstone or mica schist, both 
having surfaces favourable to the growth of the minute vegetation so much 
needed, and suitable for the attachment of zoophytes and other microscopic 
animals. When a good crop of weed has established itself, which will be 
easily known by the copious evolution of air bubbles from the coated surface 
of the stone, animal life may be cautiously added, a few sea-anemones and 
serpulz; but fish, crustacez, or any of the more aétive creatures must be 
carefully excluded from an aquarium devoted to the culture of minute animal 
and vegetable organisms. The microscopic stock finds its way into the tank 
by means of germs, which are sure to come with weed and rock, brought fresh 
from the sea. The Foraminifere, Hydroida, Polyzoa, and other minute forms, 
are not long in making their appearance, and supply the microscopist with an 
unending source of material for investigation. Microscopic slides should be 
placed in various parts of the aquarium, as minute growths developing on 
them can readily be removed for examination. Artificial aération, which is 
carried on with such great success at the Crystal Palace and elsewhere, is 
hardly needed in small aquaria containing only a few small and inaétive 
animals, but it can be obtained to a sufficient extent, by allowing water from 
a suitable source of supply to fall into the aquarium through a glass tube 
drawn out to a point; this small stream will carry with it a large quantity of 
air bubbles, and greatly aid in maintaining the purity of the water. Sea- 
water must on no account be allowed to come in contac with any portion of 
metal. 
Mr. Ingpen, Secretary of the Quekett Microscopical Club, communicated 
some notes on “ Personal Equation” as affecting microscopical observations. 
The term is a well known one in practical astronomy, and the same causes 
disturb microscopical results, although, in the latter case, they are not so well 
recognised. Mr. Ingpen called attention to the following points :— 
I. Mental equation, as causing differences in interpretation, particularly 
with regard to test objects. 
II. Nervous equation, as shown by varied sensibility to tremors, &c. 
III. Colour. Difficulty in estimating colour, as noted in Admiral Smyth’s 
‘*Sidereal Chromatics.” Right and left eye often differ in this respect. 
Effect of yellow crystalline, referred to by Professor Liebrich in his lecture on 
“Turner and Mulready.”’ Difference in visibility of violet end of spectrum, 
amounting, in some cases, to a slight fluorescence. Effect of bluish mist, 
caused by slight opacity of cornea upon estimation of the correction of objec- 
tives. Colour blindness often existing in a slight degree unsuspected, and 
difficult of detection. 
IV. Focal Equation. Differences in effect of long and short sight upon cover 
correction, &c.; also upon depth of focus, and power of resolving surface 
markings. Differences in size of images formed by right and left eye, and 
consequent effeé& upon binocular vision. Want of accommodation and pseu- 
doscopic vision, &c. 
* Where natural sea-water cannot readily be procured, the following artificial substitute 
from the formula of Mr. P. H. Gosse may be used ;— 
Chloride sodium (common salt) sete 3} ounces, 
Sulphate magnesia (Epsom salts) .. .. 4 ounce. 
Chloride magnesium .. .. .. «. «. 200 grains. 
Chloridespotassiumy 55 2 Set «a, ste | AON 4; 
Water... oe 1 gallon. 
The iodides, bromides, and some ‘lime salts found in minute proportion in sea-water are not 
included, but it is found that, when the aquarium has been established for a considerable time, 
these compounds may be deteéted. 
