E. If. NELSON ON BINOCULAR MICROSCOPES. 377 



There was another objection, viz. that the path of the rays was 

 much longer in one tube than in the other, so that two different 

 forms of eye-pieces had to be used. Very few were made, and 

 it is probable that no more will be. 



In conclusion let us examine the position of these new 

 binoculars. From what has been said above they are clearly a 

 class by themselves. It would be quite inaccurate to entertain 

 the idea that these instruments are a new kind of stereoscopic 

 binocular constructed to enter into competition with, and finally 

 to supersede, the existing binoculars of the Wenham and 

 Stephenson types ; for from what we have seen they only possess 

 the first attribute, viz. stereoscopism in a limited manner. The 

 word "limited" is used in default of a better expression. It 

 does not mean that with the Mercer effect stereoscopism becomes 

 less strong, for, on the contrary, with the Mercer effect hyper- 

 stereoscopism is often present, and care should always be taken 

 to guard against it. With the Mercer effect a cell, for example, 

 which is, and which under a Wenham would look, like an 

 ellipsoidal football will appear under a hyper-stereoscopic Mercer 

 effect as if standing on end. 



The centre of that beautiful diatom, plentiful on " Mud 

 Cuxhaven " slides, viz. Actinocyclus Ralfsii, under hyper-stereo- 

 scopism appears at the bottom of a deep pit, the outer annulus 

 being highly raised,* whereas we know that the structure is a 

 kind of shallow saucer. The word "limited" is intended to 

 apply to the stereoscopic condition that the Itamsden disc cannot 

 be centred to the pupil. The Mercer plan also entails loss of 

 light and of resolution of vertical striae. Messrs. Leitz provide 

 their inter-ocular adjustment with a millimetre scale. The 

 observer should carefully note the precise adjustment that will 

 centre the Ramsden disc to his own eyes ; half a division on the 

 scale (which represents 1 mm.) or even less ought to suffice for 

 the Mercer effect. The test of coincidence of the inter-ocular 

 with the inter-pupillary distance is that of maximum brightness. 

 Luminosity quickly falls off with either increase or decrease of 

 inter-ocular distance. With a little practice, one becomes so 

 expert in judging the luminosity that a reference to the divided 

 scale is seldom necessary. 



* Seen best with transmitted light, a No. i objective and a 1 inch 

 eye-piece. 



