PART I.] Surface Erosions on Glass Slides and Covers. I47 



in this country, ultimately converting it into a whitish translucent medium like 

 ground glass. Attention having been attracted to this as a possible explanation 

 of the previously unaccountable " milk spots," numerous experiments were tried with 

 glasses affected in various degrees with surface erosions, and a great variety of 

 extremely deceptive appearances were traced to the existence of such a condition 

 either in the cover-glasses or in the slides employed. When the erosions were 

 comparatively large, they gave rise to appearances simulating cellules of various sizes ; 

 whilst when every minute, spectral molecules, monads and bacteria were produced. 

 Some of the bacteroid markings were peculiarly deceptive, consisting of oblong or 

 rod-like appearances separated from one another by what seemed to be joints. All 

 these spectral cellules, microzymes and bacteria are of course motionless, although a 

 flight movement may sometimes appear to occur, due to imperceptible changes in the 

 position of bodies, such as blood corpuscles, in the preparation affected. 



These appearances are to be recognized in one or other of two positions in the 

 preparation, coming into focus either before, which is much more common, or after 

 the real solid bodies contained in the fluid. For example, in a preparation of blood 

 they are either found immediately beneath the cover-glass, coming into view before 

 the red corpuscles are defined, or more rarely at the bottom of the preparation after 

 the corpuscles have lost their sharpness of outline. When present in small numbers 

 only, they are frequently exceedingly deceptive, more especially those of the upper 

 layer, when, as is sometimes the case, they are interspersed among real solid particles ; 

 but there is one infallible means of distinguishing them, for all such spectra first 

 make their appearance as shaded bodies, becoming bright as the focus is deepened, 

 often, more especially when of some size, assuming a pinkish tinge whilst doing so, 

 and passing out of view as bright spaces. Now, as these phenomena are just the 

 reverse of those occurring in the case of actual solid particles, such as bacteria, which 

 appear first as bright points becoming shaded on deepening the focus, they may 

 always be distinguished by a little careful examination, but at the same time they 

 easily deceive if a preparation be but cursorily examined. Such appearances are, very 

 probably, much less liable to be met with in other climates where glass surfaces are 

 not so prone to decomposition, but the possibility of their occurrence should always 

 be borne in mind. 



This idea particularly suggests itself in connection with the importance which 

 MM. Coze and Felz attach to the occurrence of a " zone immobile " of bacteria, 

 in specimens of blood in various infectious diseases. These authors say in reference to 

 this point : — " Dans cet examen microscopique, une circonstance nous a frappes, et 

 nous n'avons trouve le fait consigne nulle part. En tournant la vis du microscope 

 pour mettre I'instrument au point, on aper9oit comme un semis de corpuscles tout a 

 fait immobiles et assez rapprochds les uns des autres. Le semis parait tantot, et le 

 plus souvent, fixd a la partie interne de la plaque recouvrante, tantot, plus rarement, 

 a la plaque inf^rieure, * * * cette zone a ^te ^galement signal^e tout r^cemment 



