874 KECORD OF CURRENT RESEARCHES RELATING TO 



therefore it becomes difficult to determine the point at whicli tlic 

 effects of the various reagents begin and end. On the other hand, in 

 the gas slide the annular channel is a mere continuation of the tube, 

 and is instantly swept clear, so that the effects of various reagents 

 mark themselves off sharply. 



Advantages of the Binocular Microscope. — Very varied opinions 

 exist as to whether the binocular Microscope is or is not of practical 

 value in histological investigations. Such authorities as Professors 

 Huxley * and Lankester in England, Professor Eanvier in France, f 

 and the German biologists almost without exception have pronounced 

 against it. Nevertheless there is undoubtedly a large class of cases 

 in which the binocular Microscope is of the greatest use in the ready 

 recognition of the true structure of an object, and this is especially so 

 in transparent objects in which the precise position of one part above 

 or below another can be recognized by means of a binocular with 

 exceptional facility, and as is said by the writer of the paper next 

 referred to, " there is no difficulty in deciding whether a fine nerve- 

 termination passes over or under or into a connective-tissue corpuscle." 

 Several cases occurred during the last scientific session in which it 

 was clear that the observers had failed to appreciate the true relation 

 of the parts of the object in consequence of the use of a monocular 

 instrument. 



Whilst, however, we object to the view that the binocular is not 

 of value in biological investigations, we have to call attention to the 

 necessity of avoiding an error of an opposite kind, viz. of laying too 

 much stress on the perfection of the stereoscopic effect when objec- 

 tives of high angle are used. An instance of this is to be found in a 

 recently published paper,| the writer of which refers to the " bold 

 relief" obtained with a Wenham prism used with an objective of high 

 angle. Cells are described as being seen, "not as flat plates, but 

 as spheroidal bodies." Now Dr. Carpenter long ago pointed out § 

 that with large angles the effect of projection is so greatly exaggerated, 

 that in the case of perfectly spherical objects the side next the eye 

 instead of resembling a hemisphere looks like the small end of an 

 egg. " Hence," he says, " it may be confidently affirmed — alike on 

 theoretical and on practical grounds — that when an objective of wider 

 angle than 40° is used with the stereoscopic binocular the object 

 viewed by it is represented in exaggerated relief, so that its apparent 

 form must be more or less distorted." 



In addition, it may be noted that high-angled objectives having 

 little " penetration " produce a false sense of stereoscopic effect by 

 reason of the parts of the object which are within and without the 

 focus being larger than those which are in focus. 



Reduction of Angle of Aperture with the Binocular. — Micro- 

 scopists should bear in mind that it is only in one direction that the 

 binocular reduces the angle of aj)erture of objectives. If for instance 



* ' Journ. Quek. Micr. Club,' v. (1879) p. 146. 



t See tliis Journal, i. (1878) p. 149. 



t ' Quar. Journ. Micr. Sci.,' xx. (1880) p. 318. 



§ ' The Microscope and its Revelations,' 5th ed. (1875) p. C9. 



