TEST-OBJECTS. 



[ 638 ] 



TETRACYCLUS. 



light, as those of Morpho (PL 1. fig. 7), of 

 Hipparchia (PI. 1. fig. 9), &c. ; as they are 

 easy tests even to inferior English oljject- 

 glasses of the present day. The long scales 

 of Pontia hi'assiccB are, however, good. 



Didymohelix (PI. 1. fig. 10 a, h, o, d). 

 The filaments should be mounted in solution 

 of chloride of calcium, or in Canada balsam. 

 It is very difficult to display the component 

 fibres of this beautiful object when in balsam. 

 It also forms a good test of magnifying 

 power. 



Didymoprium (PI. 1. fig. 11). The lon- 

 gitudinal lines upon the cells require con- 

 siderable magnifying power. 



The pygidium of Pulex. An excel- 

 lent test-object, mounted in as small a 

 quantity of balsam as possible. Dujardin 

 represents the rays upon the disks as round, 

 like so many beads, whereas they are wedge- 

 shaped with the bases outwards. 



The valves of the Diatomacece. It is 

 a difficult matter to show the lines upon 

 Grammatophora marina with an object-glass 

 of 110° of angular aperture, requiring ex- 

 tremely oblique light. 



The ultimate fibrillce of muscular fibre. 

 Mounted in liquid. KoUiker represents 

 them as beaded (PI. 17- fig- ^6 c) ; they have 

 also been represented as in a ; probably both 

 these inaccuracies arise from imperfect ad- 

 justment, and from their immersion in too 

 much liquid. Their true structure is figured 

 mb,d,f. 



Noberfs test-lines. These consist of 

 from ten to fifteen parallel bands or groups 

 of parallel lines scratched upon a slide with a 

 diamond. The bands are of equal breadth, 

 and the lines in each successive band are 

 more numerous and consequently closer than 

 those of the preceding. The breadth of 

 the intervals between the lines in the two 

 end bands is from 1-11,000 to 1-60,000". 

 The resolution of these lines forms a test for 

 angular aperture and oblique light; but it 

 can be eff'ected by a moderately good Eng- 

 lish l-8th, and is much easier than that of 

 the markings upon the valves of many Dia- 

 tomaceae. 



We have omitted to notice several test- 

 objects, as the scales of some insects, a 

 minute globule of mercury, &c. ; and this 

 advisedly, because the former have been so 

 obscurely described that we are unable to 

 comprehend in what the test-structure con- 

 sists ; and the test-appearances presented by 

 the latter viewed as an opaque object are 

 inappreciable to one unaccustomed to the 



use of the microscope, by whom mainly are 

 remarks upon test-objects required. 



Amici's test-object is Navicula gracilis, 

 the display of the lines forming the test ; it 

 is a test for angular aperture. 



Chevalier's test-object consists of the 

 scales of Pontia brassicce (PI. 2/. fig. 24), 

 the granules being rendered distinct ; this is 

 a test for definition. 



Mohl recommends the scales of Hippar- 

 chia janira for testing "penetrating" power; 

 pollen-grains, the scaly elytra of the diamond 

 beetle or bat's hair, for " definition." 



Schacht's test-object consists of the scales 

 of Hijjparchia janira (PI. 1. fig. 9 c) (a test 

 for moderate angular aperture and oblique 

 light). 



BiBL. That of the Introduction (p.xl), 

 and of Angular aperture, and especially 

 the works of Goring and Pritchard. 



TETHEA, Lam. — A genus of marine 

 Sponges. 



BiBL. Johnston, Brit. Spang, ^c. 81. 

 TETMEMORUS, Ralfs.— A genus of 

 Desmidiaceae. 



Char. Cells single, simple, elongated, 

 straight, cylindrical or fusiform, constricted 

 in the middle ; segments emarginate at the 

 ends. 



Sporangia square or round. 

 T. granulatus (PI. 10. figs. 33, 34). Cells 

 fusiform both in front and side view, ends 

 colourless and lip - like ; dots irregular. 

 Length 1-130". 



T. Icevis (PI. 10. fig. 35, in conjugation). 

 Cells in front view somewhat tapering, ends 

 truncate ; side view fusiform ; dots none, or 

 very indistinct (under ord. ilium.). Length 

 1-350". 



T. Brebissonii. Dots in longitudinal rows. 

 BiBL. Ralfs, Brit. Desmid. 145. 

 TETRABiENA, Duj.— Spores of a genus 

 of Algae, undergoing division? 

 BiBL. Dujardin, Infus. p. 330. 

 TETRACYCLUS, Ralfs.— A genus of 

 Diatomaceae. 



Char. Frustules aggregated into a fila- 

 ment, in front view broadly tabular, with 

 longitudinal uninterrupted vittae; valves 

 broadly rounded at each end, and inflated 

 on each side in the middle. 



Valves with coarse transverse striae. 

 T. Thienemanni, Ehr. {lacustris, Ralfs) 

 (PI. 13. fig. 28). Length (?). 



The structure of the compound (?) frus- 

 tules of this and many of the other tabel- 

 lar Diatomaceae, requires careful investiga- 

 tion, for the valves formed by division appear 



