1886.] 



MICKOSCOPICAL JOURNAL. 



27 



supply moisture, but an additional 

 one with a larger hqle at the centre 

 facilitates the removal of the cover) . 

 The other plate should have a larger 

 central perforation, over which a ring 

 and cover-glass are cemented. When 

 in use this one is placed over the for- 

 mer, covering the object with the 

 cell, and the wdiole placed over a 

 small dish of water, with the tongue 

 reaching the water. It will be seen 

 that an examination with a low power 

 mav be made at any time through the 

 cover ; the cover to be removed for 

 the use of high jDOwers. 



After all, how much better are 

 these devices, or any devices, of the 

 kind used onlv for interrupted obser- 

 vations, and in which the object is 

 retained in a limited supply of water, 

 surrounded by moist air. to simply 

 placing the object on a slide, and cover- 

 ing bv a bottle that has been cut oft', 

 and the edge ground on a plate of glass 

 v^ith a little emery and water. If the 

 ground edge has been smeared with 

 tallow, and some moistened blotting- 

 paper put in the top of the bottle, the 

 drop remains without loss, and is 

 ready for examination by setting oft' 

 the cover. 



Microscopical Advances — Ancient 

 and Modern.* 



BY G. W. ROYSTON-PIGOTT, M. A., 

 M. D., CANTAB. 



In nothing has the jjrogress ot mi- 

 croscopical powers been more pleas- 

 antly displayed than in the gradual 

 development of the precise optical 

 definition of dots and spherules. 



In copying with the camera lucida 

 at a power of 5, I was greatly struck 

 with Chevalier's representation of the 

 'Podure,' which was executed by 

 means of la chambre claire. In' a 

 talented young artist about 1S39. 

 Pritchard speaks scornfully of these 

 French poduras as being too easy. 

 These scales (the present Degeeria 

 domestica) ^ a.^ shown by Chevalier, 

 are marked with black dots linearlv 



*From a series of articles in 7 he E>!i>/ish Mechanic. 



arranged, and in other scales with 

 double rows of dots, nearly parallel, 

 and crossing each other at an acute 

 angle. The contour of the scales in 

 the steel engraving is decidedly oval. 

 M. Chevalier was the first to give this 

 resolution, which he does as fol- 

 lowing (translation) : — ^ Podure. 

 The scales of the Podura havegeneral- 

 Iv an oblong form, but are of various 

 sizes ; with a mediocre microscope 

 their surface appears blank, but with 

 a perfect instrument one discovers an 

 infinite number of points oblong, 

 which imitate straight lines, crossed, 

 oblique, or wavy, following the 

 changes of the illuminations. It is 

 not very difficult to discover these 

 points upon the largest scales. It is 

 necessary to choose the smallest, and 

 we consider them as the best test ob- 

 jects to show the penetrating power 

 of the microscope.' 



I was not aware of the existence of 

 this plate in 1869, when I first pub- 

 lished resolution of this scale. The 

 merit of this microscope is show^n by 

 the award of the gold medal to 

 Chevalier by the jury, who declare 

 that ' we have compared it with an ex- 

 cellent microscope of Amici, the best 

 of those we possess in Paris, and that 

 we recognize, not with astonishment, 

 but with a lively satisfaction, that the 

 microscope of Mr. Charles Chevalier 

 is truly superior to that of Amici.' I 

 find Chevalier's plate of the Lepisma 

 is most exquisitely formed of a double 

 set of radiating short lines of dots 

 (denied by Dr. Carpenter). 



The English resolution of the Po- 

 dura is still like a series of oat seeds 

 arranged somewhat linearly, and the 

 sharpness of the outlines of these 

 markings is regarded as a most severe 

 test for the objective quality (as far 

 as it goes) . Pritchard's Hair of Der- 

 mestes demonstrates at sight the pro- 

 digious advance the microscope has 

 made. A minute translucent cup, 

 ornamented with four petals and in- 

 tense black lines, marks the interfer- 

 ences of tissvies stopping the passage 

 of light, showing transcendent defini- 



