22 On the High-Power Definition of 



shadow. It is not many years since I witnessed a skilful micro- 

 Bcopist manipulating in the most tedious manner before he could 

 show an object, now easy. Then arose " the kettledrnm " of jMr. 

 Eeade and the Prism. 



From studying the history of definition, we have that of the 

 microscope; and ive see how good shadows really enabled a had 

 glass to j^erform lil-e a good one. And this was designated by the 

 high-sounding title of microscopical skill. 



What histories in histology are connected with the centkal 

 STOP ! now discarded. It was much the same thing whether the 

 central stops were placed in the condenser or placed within the 

 objective. Looking back on those feeble microscopic powers, we 

 calmly recognize that it was the disagreement of the central rays 

 with the peripheral, or the bad qualities of the glasses which were 

 concealed by cutting off the central rays. A black spot, actually 

 produced by the black central stop, was really believed to be the 

 proper definition of the diatom long before beads were demon- 

 strated. A central artificial shadow imposed upon the credulity 

 of all. 



"We never could make anything of the structure of the surface 

 of the moon, without watching shadows under difierent degrees of 

 obliquity. At four, six, and ten days' age, these shadows gradually 

 steahng forth reveal the noble ruins of extinct volcanoes in all their 

 marvellous beauty and distinctness. In the same manner studying 

 an object under gradually deeper or more slanting shadows gives 

 an idea of contour and configuration that nothiag else can. 



Several facts are noticeable. 



A rouleau of beads lit up obhquely gives under imperfect resolving 

 powers short straight lines Uke the rounds of a ladder, as in Fig. 1, 

 Plate XXIII. ; more perfectly they are seen as closely-packed black 

 crescents. If the position of these crescents change as the object is 

 made to revolve, and change symmetrically, spherical bodies may be 

 predicated, though invisible. Again, a changing lattice-work, one 

 set appearing to pass and repass across the other as we move the 

 light, indicate ribs in two planes slightly incHned to one another. 

 The violent attempts made to define the then difficult structures 

 were seen, as just remarked, in the necessity of illuminating by two 

 lamps or other contrivances in two distinctly difi'erent planes at one 

 and the same time. With our improved glasses this now appears 

 puerile. But still two sets of shadows were obtained, and this 

 enabled the better rays of the glass to present an image to the eye. 



The most difiicult and delicate observations are now successfully 

 made with nothing but central light : a pin-hole in a cap over the 

 condenser : or with unilateral light ; and even with no cap at all. 



It is a strong fact that in every case two sets of lines, if made 

 out, really heralded beading. 



