190 



♦ KNOWLEDGE ♦ 



[March 30, 1883. 



not the least faithful, or lo\inj,', or respectful of his people 

 would refrain from asking for any such concessions, would 

 even take ploasiirc in telling him that in their full assurance 

 of the perfect wisdom of his ordinances, they could 

 not ask for anything l>ut that his will should be fulfilled. 

 Now, without asking which of these two classes of this 

 ruler's people would be the wiser, I think 1 may safely assert 

 that neither of them could reasonably look askance at the 

 other as unloving subjects and an unbelieving, untrusting 

 people. If, then, to apply the parable, anything I have here- 

 tofore said on this subject may seem to imply that I regard 

 those who honestly make " prayers which have to do with 

 nature " as showing want of faith in God's wisdom and in 

 His knowledge of their wants, 1 now definitely say that such 

 doubts would be unfair and unreasonable. But, on the 

 other hand, they should not regard those who find reason 

 (as I must confess 1 do) for refraining from appeals of that 

 particular kind, as therefore having no belief in God and 

 no faith in His power. We believe the laws of nature to 

 be unchanging, because they are so perfect as to need no 

 change, though, in individual cases, their operation may 

 seem to our limited knowledge to be mischievous or hurtful. 

 Contest between those who see either the silver or the 

 golden side of the shield alone, is beyond question a great 

 and mischievous mistake. 



PLEASANT HOURS WITH THE 

 MICROSCOPE. 



Bv Henry J. Slack, F.G.S., F.RM.S. 



BESIDES colour-blindness, which is an organic defect, 

 there is an immense deal of insensibility to colour 

 arising from want of practice with the eye and the brain. 

 A majority of uncultivated people never see many of the 

 qxiieter colours with sufficient discrimination to be in any 

 way impressed liy them, or to be able to recognise any 

 differences in those which lie near each other in the colour 

 scale. In actual landscape or in works of art, few persons 

 who have not taken some pains in learnLiig to see, notice 

 half as much as strikes an educated eye, and as the public 

 contains a large proportion of imperfect seers, artists often 

 obtain great praise for scenes in which they have omitted 

 the most beautiful and characteristic chromatic eliects ; or, 

 if they happen to paint with a wider truth to nature, are 

 told by the critics that they are quite wrong. The micro- 

 scope when furnished with polai-ising apparatus offers the 

 readiest means of studying an endless variety of colour 

 contrasts or combinations, and might be of gi-eat use to 

 artists and decorative designers. 



A few examples will indicate what to do. We will begin 

 with one of the most useful substances for these experi- 

 ments— salicine. A few grains of this substance should be 

 dissolved in water, and a number of glass slides should 

 have a drop of the solution, in various strengths, put upon 

 them. Some should have the drop evaporated quickly over 

 a lamp, others more slowly, and others left to spontaneous 

 evaporation. By practice, a great variety of crystallised 

 pictures will he obtained. Th(! normal form is that of a 

 circular group of crystals, radiating from a centre, and all 

 in approximately the same plane, though not quite 

 30, on account of variation in the thickness of the 

 needles. The chromatic effects obtainable with the 

 polarising apparatus result from interferences of the light 

 waves analagous to what occurs with sounds when two or 

 more of different pitch are superposed. The slightest 

 difference in the thickness of the cry-stals that act upon 

 the polarised light gives a diirerence of colour or tint Our 



first experiment is made by placing our polarising and 

 analysing prisms across each other, so that a dark field is 

 obtained. Tho polari«er is placed under the stage of the 

 microscope, and the most convenient place for the 

 analyser is just above the object-glass, and with an 

 arrangement that allows of its rotation. Any optician 

 will show and explain this. Daylight is best for accurate 

 exhibition of all the obtainable colours, and the observer 

 should be a little in the shade, by the side of the window, 

 with the microscope so placed that its sub-stage mirror can 

 easily catch the light. At night, an ordinary paraftin|^lamp 

 answers well, but its light is a little yellower than day- 

 light, and consequently cannot show some colours. For 

 most purposes this does not matter. 



When the field is darkened through the positions of the 

 prisms preventing the transmission of polarised light, a 

 change takes place as soon as the salicine slide is placed on 

 the stage. The circular crystal groups will most likely 

 vary in size and colours. One, for example, in a slide now 

 under view with a l.',-in. objective and its eye-piece, looks 

 the size of a shilling, and it exhibits radiating bands of 

 yellow, greenish-yellow, a rose violet, a dark brown, and a 

 little brilliant blue. Another group, twice as large, 

 shows contrasts of blues, yellows, greens and lirowns, 

 but no warm reds — or, indeed, any reds. A quarter 

 turn of the analyser changes the soene, and gives 

 blues, reds, and violets, contrasting with white spaces. 

 The effect cannot be called a colour discord, but it is not 

 agreeable. What is the matter 1 Substituting a 4 in. 

 for the liin. objective at once explains. The field is 

 now much larger, more of the cii-cular gi'oups can be seen 

 at once, and each one is so much smaller that its colour 

 stripes lie closer together, and the whole effect is less 

 violent. The impression is now quite satisfactory, and 

 something like it would do well in decorative art Using 

 different slides will show to what an extent relative propor- 

 tion and actual size both have in determining whether a 

 colour composition, not essentially wrong, shall be beautiful 

 or displeasing. 



The most charming effects are obtained when a crystal- 

 line substance is induced by the sort of treatment already 

 mentioned to do as ice crystals do on our windows — 

 depart from angular and run into curved floral patterns. 

 The slide under examination gives, in one part, an exqui- 

 sitely quiet design of blues, yellows, and browns, with 

 numerous other tints, the whole suggestive of repose. 

 A slight movement of the analyser makes the same crystals 

 give a rather irritating pattern of blues, yellows, greens, 

 and some little reds. By moving in rotation, sometimes 

 the polariser, and sometimes the analyser, in various 

 degrees, a great variety of experiments may be quickly 

 made. 



For another set of observations we will use, besides the 

 polariser and analyser, some thin sHces of selenite, called 

 Darker's films. If the observer has not got these, he can 

 make thin slices of mica answer many of their purposes. 

 The apparatus in use at this moment allows the films to be 

 rotated. They are now arranged to give a yellow field, 

 much like pale primrose, which does not fatigue the eye. 

 The whole slide, when introduced, is now beautiful, but in 

 most parts too powerful in effect for steady contemplation. 

 Some parts, however, are soft enough, and we find this modu- 

 lation is effected by diminishing the size of the brilliant-tinted 

 parts, and introducing more greens and blues. A mauve 

 ground is obtained by another change in the position of 

 the films. This makes the larger crystal groups too 

 violent, but is soft and Indian shawl-like with some of the 

 smaller floral groups. A very pale mauve ground, 

 obtained with another film, succeeds admirably, and 



