56 NOTES AND MEMORANDA. 



well, but it does not entirely prevent the evaporation of the liquid in 

 whicli tlic objects are contained. The ordinary concave slide, though 

 better than a plain slip of glass, does not fulfil all the requirements ; 

 and with such a slide it is difficult to keep the object in focus, except 

 with very low powers. 



To obviate these difficulties, Mr, Weber has reversed the form of 

 the cell, and forms his slide as shown in the accompanying engra- 

 ving, where A is the convex bottom of the cell, and B the thin glass 



cover, a drop of water being held between them by capillary attrac- 

 tion. When the cover is cemented down by means of a little water- 

 proof cement, the water cannot evaporate, and the whole arrangement 

 forms an air-tight aquarium on a minute scale. The open space forms 

 a chamber which retains a supj^ly of air, and if the animal and 

 vegetable life are properly balanced, life may exist in one of these 

 slides for weeks. 



In the woodcut, which shows the slide, the thickness of the slide, 

 &c., is magnified about four times.* 



The form of the crystalline Cones in the Arthropod Eye. — Oscar 

 Schmidt contributes a paj)er on this subject to the ' Zeitschrift f. wiss. 

 Zool.' t He commences by a short statement of the views of Exner 

 and of Grenadier, the latest writers on the subject, and remarks that 

 both they and all their predecessors consider each visual cone to be 

 perpendicular to the corresponding corneal surface, so that only those 

 rays of light which strike the cornea at right angles are of any avail 

 in the formation of an image, being able to pass unbroken and unre- 

 flected to the apex of the cone. 



The author then proceeds to describe the visual rods in the amphi- 

 podous genus Phronima. This animal possesses two pair of eyes : 

 lateral eyes (Seiten-augen), situated in the usual position at the sides 

 of the anterior part of the head, and in the same transverse section 

 as the brain ; and frontal eyes (Stirn-Scheitel-Augen), placed at the 

 vertex of the head, very far posterior to the brain. l'"ach eye is supplied 

 with nerve-fibres from the optic ganglion, which fibres enter a mass 

 of pigment of a brown or yellow^ colour visible with the naked eye, and 

 become surrounded with a sheath closely adherent to the pigment. The 

 pigmented body of the lateral eye is comj)aratively small, that of the 

 frontal eye largeand spindle-shaj)ed; withit are connected, in each case, 

 the proximal ends of the visual rods or crystalline cones, the distal ends 

 of which abut against the external surface. In the lateral eye no two 

 cones are found alike : those of the central portion of the organ 

 approach most nearly to the conical form, and even they are not really 

 conical, having an almost globular distal extremity or head, and a 

 spindle-shaped swelling near the proximal end, where they become con- 

 tinuous with a nerve-fibre. In the frontal eye the visual rods are 



* ' American Journal of Microscoi^y,' vol. iii. p. 253. 

 t ' Zeitsch. f, wiss. Zool.,' vol. xxx. (Suppl.), p- 1- 



