2S4 1 APPARATUS FOR EXHIBITING COLOURS, 



Methods of pro* The coloursof thin fubftances muft often have been feen in 

 low". 5 " Du bbJ es of water or of other fluids, and in the film produced 



by a drop of oil fpreading on water ; they were more particu- 

 larly obferved in the plates of talc, or of felenite, into which 

 thofe fubftances readily divide. Sir Ifaac Newton made his 

 experiments principally on the colours of foap bubbles, and on 

 Mr. Jordan's thofe which are produced l>y the contact of two Ienfes. For 

 thod of exhibit" "ifpe&ing the colours of foapy water, the moft convenient 

 ing the foapy method is that of Mr. Jordan. He dips a wine glafs into a 

 * a * weak folution of foap, and then holds it in a horizontal posi- 



tion againft an upright fubftance, for example, a window 

 mutter ; the film covering the glafs being in a* vertical pofi* 

 tion, the gravity of the fluid tends to make it thicker at the 

 lower part, and it becomes every where gradually thinner and 

 thinner, till at length it burfts at the uppermoft point. The 

 colours affume, in this cafe, the form of horizontal ftripes, 

 fimilar to the rings which are to be more particularly de- 

 {bribed. 

 The coloun It has been obferved by Newton, that the colours thus re- 



tiorn a en e fl e &ed from a plate of a denfer, medium, are more vivid than 

 vivid than thofe when a plate of a rarer medium is interpofed between two 

 iTTufe^he'latV Center mediums. But the caufe of this apparent difference is 

 tcr are diluted by probably, the quantity of foreign light that is generally prefent 

 ? A^ h V C " v„ m tne experiment, reflected as well from the upper furface of 

 coining me- the fuperior medium as from the lower furface of the inferior, 

 d.ums. bot.ii thefe fur faces being often nearly parallel to the furfaces 



in contact. It becomes therefore defirable to remove this fo- 

 Remedy. reign light : this may be done effectually, by employing one 



glafs in the form of a prifin, and coating the lower furface of 

 the other with black fealing wax : the light reflected by the 

 oblique furface of the firft, is thus thrown into another direc- 

 tion ; and the reflection of the inferior furface of the fecond, is 

 either deftroyed or rendered imperceptible. And, with thefe 

 precautions, the rings of colours produced in the reflected 

 light, may be rendered a very beautiful object by means of the 

 folar microfcope. 

 The plane glafs The molt perfectly plane glaffes are thofe which are ufed for 

 u made pnfma- £i ac ii e y } s quadrants : one of thefe may be ground in the di- 

 lowerfu.face of re&ion of the diagonal of its tranfverfe fection, foas to make a 

 the lens coated fa\ n wedge or prifrn ; and the furface of the lens employed 

 muft be a portion of a fphere of from five to ten feet radius. 



The 



