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For your situation, there are two practical options:

1) You could use a thinner coverslip and immersion oil with higher 
refractive index than standard oil.  Wan DS, Rajadhyaksha M, Webb RH 
(2000) Analysis of spherical aberration of a water immersion objective: 
application to specimens with refractive indices 1.33-1.40. J Microsc 197: 
274-284, see also comments by Sheppard J Microsc 200: 177-178
The problem is that for full correction at 1.25 NA, you may need immersion 
oils of very high RI- those may not be available.  

2)
Use a finite-corrected objective on an infinity-corrected system: REIHANI, 
S.N.S. & ODDERSHEDE, L.B. (2009). Confocal microscopy of thick 
specimens. Journal of Biomedical Optics 14, 030513-030513-030513.
 

You could also change the tube length to compensate for spherical 
aberration - on some old compound microscopes with finite optics and a 
trinocular tube it is easy, you can move the camera up or down from the 
default 160 mm tube lengths position. I am not sure how easy it would be 
on some newer models; probably not possible or practical or modern point 
scanning confocals.

Stan Vitha
Micrsocopy and Imaging Center
Texas A&M University 

On Tue, 5 Apr 2016 01:39:45 +0000, MODEL, MICHAEL 
<[log in to unmask]> wrote:

>
>I  think if you attempt to use a lower refractive index you will be getting 
additional aberration from a coverslip. To test the effect of oil, you can 
attach beads to two coverslips and place then facing each other and 
separated by a gap of appropriate size (Journal of Microscopy, Vol. 241, Pt 
1 2011, pp. 94–100). Then compare the quality of images of the top an 
bottom beads.
>
>Mike 
>
>________________________________________
>From: Confocal Microscopy List 
<[log in to unmask]> on behalf of Knecht, David 
<[log in to unmask]>

>If you are imaging a sample in aqueous media through glass with a 1.25 
NA 100x oil objective, what is the correct refractive index immersion oil to 
use?  You are going to have refractive index mismatches no matter what, 
but are some more critical than others?  Is the 1.25 NA objective designed 
to be used with 1.515 RI oil or should one use a lower RI oil or match the 
sample by using water?  Dave
>
>Dr. David Knecht