See the previos page of this forum, thread "Continious interpolation in excel" Thread770-13682.

alternatively, the data you provided seems to fit pretty well on this curve:

y = -0.0114x^2 + 34.782x - 26554

where y is dispersion and x is wavelength, in whatever units you used for your posting.

I used the formula you suggested an the other thread and it worked fine.
I can't use the fit you suggested since I use more datapoints in the real dataset and a fit doesn't give values that are correct enough.
Thanks a lot for the help.
By the way is it possible to define a makro or function that will do the same and where I would just have to specify the dataset I want to interpolate and the value it should be interpolated at.

Jacob

Jacob,
you can rewrite the formulas in the worksheet as a VBA function and use it straight from the worksheet

Public Function INTER1(Xval As Double, X As Range, Y As Range)
Dim Nrow%

Nrow = Application.WorksheetFunction.Match(Xval, X, IIf(X(1) > X(X.Count), -1, 1))
INTER1 = Y(Nrow) + (Xval - X(Nrow)) / (X(Nrow + 1) - X(Nrow)) * (Y(Nrow + 1) - Y(Nrow))

End Function

Thanks

Jacob

Although previous responses will achieve results, there are standard Excel functions that will accomplish the same result.  In this case, the function is the "linest" function.  Please note the following:

Disp    wavelength        
-76    1554        
-77.5    1556        
-79.1    1558        
-80.8    1560        
-82.5    1562        
-84.3    1564        
-86.2    1566        
-88.2    1568        
-90.2    1570        
-92.4    1572        
-94.7    1574        
-97.1    1576        
-99.6    1578        
-102.2    1580        

-0.011158855    -2.972701541    1392.636695    #N/A
0.000305148    0.054237082    2.393678956    #N/A
0.99994583    0.066942506    #N/A    #N/A
101527.5568    11    #N/A    #N/A

=LINEST(D5:D18,C5:C18^{1,2},,TRUE)

The array shown above, with results, is an array formula to determine coefficients of 2nd degree polynominal.  The equation listed is the array formula, where column D are y-values and column C are x-values.  The first row of results are the coefficients (A2, A1, A0) for the 2nd degree polynominial.
FYI, Excel does have functions to determine coefficients of equations for a specified data.  Please read the Online Help regarding the Linest Function.  Not included in online help is method or technique to determine coefficients for various types of equations.
For this situation, data was plotted and a quadratic or 2-degree polynominial curve fit will suffice.  If not 2-degree, then simply add a ",3" (without quotes) after the "2" in linest function for a 3rd degree polynominial.
Enter the linest function as an array formula.  That means to: 1) highlight or select the range of cells (this case 4 rows x 4 columns) where formula is placed, 2) type in or use function wizard for linest function, using technique above, and 3) pressing ctrl+shift+enter.  See above example.  It is better to utilize the standard features of Excel than to writing functions to accomplish the same tasks.
I trust this helps.

pmover,
Thanks for th insights of LINEST() function, I had no idea about its existence. The LINEST() Returns an array that describes a straight line that best fit your data, calculated by usnig the least squares method. The VBA function in the my earlier reply simply returns linearly interpolated value between two data points. Two different goals...