TEST OBJECTS. 473 



Scak of Morpho Menelaus. — A scale of this splendid butter- 

 fly is shown at fig. 4, plate viii., magnified 500 diameters ; it 

 exhibits strongly marked longitudinal and very delicate trans- 

 verse striae, the former frequently bifurcating. In former times 

 it required a good quarter to exhibit the transverse striae; 

 but the half-inch, as now constructed, wiU show them readily. 

 In taking the scales from the upper surface of the wing of 

 this beautiful insect, the pale blue specimens should be se- 

 lected ; many of these have a thick coating of colouring matter, 

 and in examining a series of them, it will often happen that 

 scales will be seen having certain spaces or parts of their 

 surfaces more transparent than the rest, and without any 

 trace of strise ; this is due to the removal of the pigment, and 

 with it the striated layer. This object forms a good test for 

 the half-inch object-glass, which should show clearly the 

 transverse striae ; and if the scale be perfectly flat, the strife 

 should be seen over the whole of its surface ; but it generally 

 happens that they are only well defined in certain situations. 

 The pigment under very high powers exhibits a dotted ap- 

 pearance between the strise. 



For many years several species of siliceous infusoria of the 

 genus Navicula have been employed as tests ; but with im- 

 proved object-glasses the lines or dots on their surfaces can be 

 so easily made out, that they are no longer used; but in 

 more modern times, several new species have been discovered, 

 which even now require the aid of the highest powers and 

 most careful manipulation to show their true characters ; the 

 first of these, and the one most easily exhibited, is the Navicula 

 Hippocampus. This beautiful species was first brought under 

 the notice of the microscopists in this metropolis, by Mr. 

 Robert Harrison, of HuU, in June, 1841, the longitudinal 

 strife, on the surface of which, he was the first to discover. 

 After a careful examination of the same infusoria at a sub- 

 sequent period, Mr. Harrison also detected transverse strite as 

 well; but these he found more diflScult to exhibit than the 

 longitudinal series. A representation of this animalcule is 

 given in plate ix., fig. 1, as seen under a magnifying power of 

 500 diameters, and at fig. 2, under one of 1,200. 



