COKRESPONDENCE. 



151 



"Audi Alteram Partem." 



To the Editor of the ' Monthly Microscopicul Journal' 



Sir, — I thank you for inserting my letter (in your last number, 

 page 100) upon the new, or immersion, mode of using achromatics. 



But, as there are always two sides to every question, it is but fair 

 and right to state what may be said on the other side. 



And first, it is a decided objection that the interposed drop of 

 water greatly prevents our judging of the actual distance of the outer 

 lens from the covering glass ; and in consequence of this, very vexa- 

 tious accidents may happen — exempli gratia, one of my first essays with 

 the new plan was upon the " Diaiomaceen Typen Platte " ; and I would 

 caution those who possess that somewhat costly little curiosity to take 

 care how they examine it with the " aquatic " ; for not being, then, 

 aware how much the focal distance was increased by the new mode, 

 and it being, moreover, difficult to know exactly hoiv near the glass 

 was, for the reason already stated, I natui'ally thought the glass was 

 without the focus, when it really was ivithin it ; and therefore turned 

 downwards, very " gingerly," until I was suddenly appalled by a 

 horrid crash ! and, on examination, found to my sorrow that the 

 " aquatic " had thrust his " nozzle" right through the covering glass ! 

 And this leads me to observe that the said Typen Platte is not made 

 in the same way as our English diatom slides are. The latter are 

 made by merely placing the diatoms on the slide, pouring on a little 

 balsam, and then, having indurated it by heating over a spirit-lamp, 

 the covering glass is pressed down upon it ; and thus the slide, balsam, 

 and covering glass become as though one solid mass ; and such a slide 

 will bear to have the object-glass turned down upon it, until the safety- 

 spring (with which all good microscopes are furnished) yields, and so 

 gives notice that there is contact. 



But let everyone beware of doing this with the Diatomaceen Typen 

 Platte. The latter, I understand, is made by " sjunning " a ring of 

 balsam upon the slide. Then, when that is hardened, some very thin 

 balsam or varnish is poured on ; and the covering glass (to which the 

 diatoms have been previously attached) is gently placed uj)on the afore- 

 said ring. Now, by this mode, there is a space between the slide and 

 covering glass which is filled with balsam, or some equivalent, in 

 nearly a fluid state ; and the covering glass being about as thin as a 

 bank-note, it will not bear the slightest contact of the achromatic with- 

 out destruction ! Under this head I may mention one important fact. 

 When the aforesaid accident has occurred, and the unfortunate " Platte " 

 appears " lost beyond rederai^tion," it may still, in a measure, be re- 

 stored in the following manner : — 



Provide a disk of the thinnest glass, of the exact size of the outer 

 glass of the Platte (which is four-eighths and a sixteenth, English 

 measure), and then, having dropped a little hot and very fluid balsam 

 upon the fractured glass, gently press on the new cover. This will 

 heal ujD the cracks, rendering them nearly invisible ; and the " Platte" 

 takes a " new lease of life." 



I would willingly end here; but should like to state objection 



VOL. X. N 



