86 



HAHDWICKE'S SCIENCE-GOSSIP. 



regulated by the pressure of a screw acting against a 

 spring, at the end of which the prism is fixed. By 

 slightly turning this screw, the prism may be regu- 

 lated so as to throw an exactly equal amount of 

 light up the two eye-tubes. Your correspondent 

 says he sees a " dark line " across the field ; and 

 then says he finds the outer halves of the field dark. 

 The latter is the fact. The high, powers, with com- 

 mon prism, exhibit only the central part of the field, 

 and obliterate the sides. Thus, when I use a com- 

 mon Hh with common prism, I have only a narrow 

 strip of light through the centre of the field ; but 

 when I remove it and adopt tlie ith with hdernal 

 prism, all is perfectly right. In short, if your cor- 

 respondent wishes to use his \ with the common 

 prism, he must have the tube greatly shortened, 

 so as to bring the prism and lenses as near together 

 as possible. Mr. Wenham, the inventor, long ago 

 fully explained all this in the Monthly Microscopical 

 Journal. — /. 



Dark Lines in Tjeld of View.— "J. W." would 

 find a condenser, and a piece of fine ground glass 

 below the slide, would remedy the defect com- 

 plained of, and add very much to the stereoscopic 

 efi'ect. My own condenser consists of a plano- 

 convex lens of i-inch focus, and a brass cap screw- 

 ing over it, to which a disk of ordinary thin glass 

 (ground on the under side) is attached by means of 

 marine glue or asphalte varnish. The thin glass 

 I grind on a piece of plate-glass and flour of emery, 

 mixed with a little water. By this means I can use 

 a \ objective without withdrawing the prism, and 

 both tubes are fairly illuminated. — F. K. 



Thiersch's Cement for Objects mounted in 

 Resinous Substances.— Dissolve shell-lac in spirit 

 of wine, in sufficient quantity to make a thick 

 varnish; colour with a concentrated solution of 

 aniline-blue or gamboge in absolute alcohol. Add 

 about a scruple of castor-oil to each ounce of the 

 mixture. After some further evaporation, it must 

 be preserved in a well-closed vessel. Previous to 

 using this cement, the inventor directs that the 

 edges of the covers of balsam-mounted slides 

 should have a layer of balsam dissolved in chloro- 

 form put round them, in the same manner as 

 asphalte ; and at least three days, but, still better, 

 weeks or months, should be allowed to elapse before 

 applying the cement. {The Microscope, by Dr. 

 Prey.) (I have used for some time dammar solu- 

 tion, coloured with vermilion, zinc-white, emerald- 

 green, &c. If the object is mounted in soft balsam 

 or dammar, a layer of ordinary shell-lac varnish 

 should be previously applied. If the object is 

 mounted in a thick cell, the angle between the 

 cell-wall and slide can be filled up with whiting, 

 mixed with gum-water, and the coloured varnishes 

 afterwards applied. — F. K.) 



Blood Crystals and Hydro-chlokate of 

 H^metin. — Bojanowski recommends the follow- 

 ing plan to procure the ordinary form of blood 

 crystals. The blood should be taken from the 

 vessels of a dead animal, and kept in a vessel two 

 to four days in a cool place. A drop of this fluid is 

 to be put on a slide covered, and exposed to the 

 light for a few hours. Tae crystals may then be 

 seen. The formation of crystals takes place readily 

 in electrified blood ; in that, from the guinea-pig 

 their formation is so rapid that they appear as 

 though struck out by the spark. Bottcher says 

 that chloroform, with access of air, produces blood- 

 crystals. Lehman gives the following directions for 

 the production of crystals of hydro-chlorate of 

 ha:metin : — Presh blood should be treated with 

 alcohol, containing oxalic acid and ether (1 part 

 alcohol, 4 parts ether, and i^tb of a part of oxalic 

 acid). Preserved in well-closed bottles, the crystals 

 are gradually precipitated : the process is hastened 

 by the addition of chloride of calcium, liquefied by 

 exposure to air. When separation takes place 

 quickly, the crystals are more or less acicular ; if 

 slowly, they are either hexagonal, or long, narrow, 

 and laminated, and appear sometimes twisted on 

 their long axis.— The Microscope, Dr. Frei/. 



Cements. — There is a neglected varnish, which 

 I should like to recommend to your readers who 

 mount their own objects; namely, copal. It can 

 be bleached by exposure to the light for a few 

 months, and when old and good, makes a cell for 

 liquids or glycerine, which is so clear that it looks 

 like a balsam slitle ; and, when allowed time to set 

 before using, as near imperishable as can be 

 needed. One point should be observed in this, 

 as in other mounts, I think. Whatever cement be 

 used for the cell, the same should do for the 

 covering and finishing. If a picture were laid in 

 with one vehicle and finished with another, it 

 would certainly crack all over some time or other, 

 owing to the different rates at which different 

 media absorb oxygen. I have cells of copal, deep 

 and shallow, glass rings, &c., cemented with it, 

 which show no deterioration since 1861.— <?. 7?^. 



Mounting Diatomace^. — Your correspondent 

 C. L. Jackson will find the mounting of diato- 

 macese and other minute objects in balsam greatly 

 facilitated by the use of a little instrument devised 

 by Mr. James Smith, and supplied by Mr. Collins, 

 of Great Portland-street, London. This consists 

 essentially of a small brass plate turned up at the 

 edges, on which to lay the slide, a handle to hold it 

 by, and an ivory knob, so arranged on a lever that 

 it can be brought down on the coverinp,- glass with 

 a steady graduated pressure, by means of a fine 

 screw. This instrument may be used as follows : — 

 The slide, with the objects arranged on it in any 

 pattern that may be desired and covered with a 



